Unknowns mount in search for jet

• Despite efforts of 26 nations, no sign Monday of missing airliner


• "Surely, they must find the plane," father of missing passenger says


• As theories mount, Malaysian officials plead for end to speculation


• Theories "distract us from our immediate concern," aviation chief says

(CNN) -- The search for missing Malaysia Airlines Flight 370 entered its 11th day Monday, saddled with the growing weight of disparate and sometimes conflicting theories as to what might have happened and where the plane might be.

Search crews from 26 nations scoured vast swaths of ocean and land for any trace of the airliner, which vanished March 8 on a flight between Kuala Lumpur, Malaysia, and Beijing.

They've turned up nothing.

Meanwhile, families struggle with the unknown.

Partner: I have to prepare for worst

Did plane drop 5,000 ft. to avoid radar?

See new video of Flight 370 pilot

Relatives of passengers aboard Malaysia Airlines Flight 370 watch a news program about the missing plane as they wait for information at a hotel ballroom in Beijing on Monday, March 17. The Boeing 777 disappeared during a March 8 flight from Kuala Lumpur, Malaysia, to Beijing.

Malaysian Transportation Minister Hishamuddin Hussein, center, shows maps of the search area March 17 at a hotel in Sepang, Malaysia, next to the Kuala Lumpur International Airport.

U.S. Navy crew members assist in search-and-rescue operations Sunday, March 16, in the Indian Ocean.

Indonesian personnel watch over high seas during a search operation in the Andaman Sea on Saturday, March 15.

A foam plane, which has personalized messages for the missing flight's passengers, is seen at a viewing gallery March 15 at Kuala Lumpur International Airport.

A member of the Malaysian navy makes a call as his ship approaches a Chinese Coast Guard ship in the South China Sea on March 15.

A Indonesian ship heads to the Andaman Sea during a search operation near the tip of Sumatra, Indonesia, on March 15.

Elementary school students pray for the missing passengers during class in Medan, Indonesia, on March 15.

Col. Vu Duc Long of the Vietnam air force fields reporters' questions at an air base in Ho Chi Minh City, Vietnam, after a search operation on Friday, March 14.

Members of the Chinese navy continue search operations on Thursday, March 13. The search area for Flight 370 has grown wider. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, efforts are expanding west into the Indian Ocean.

A Vietnamese military official looks out an aircraft window during search operations March 13.

Malaysian air force members look for debris on March 13 near Kuala Lumpur.

A relative of a missing passenger watches TV at a Beijing hotel as she waits for the latest news March 13.

A member of the Indonesian National Search and Rescue Agency scans the horizon in the Strait of Malacca on Wednesday, March 12.

Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12.

Journalists raise their hands to ask questions during a news conference in Sepang on March 12.

Indonesian air force officers in Medan, Indonesia, examine a map of the Strait of Malacca on March 12.

A member of the Vietnamese air force checks a map while searching for the missing plane on Tuesday, March 11.

Iranians Pouri Nourmohammadi, second left, and Delavar Seyed Mohammad Reza, far right, were identified by Interpol as the two men who used stolen passports to board the flight. But there's no evidence to suggest either was connected to any terrorist organizations, according to Malaysian investigators. Malaysian police believe Nourmohammadi was trying to emigrate to Germany using the stolen Austrian passport.

An Indonesian navy crew member scans an area of the South China Sea bordering Indonesia, Malaysia and Thailand on Monday, March 10.

Vietnam air force Col. Le Huu Hanh is reflected on the navigation control panel of a plane that is part of the search operation over the South China Sea on March 10.

Relatives of the missing flight's passengers wait in a Beijing hotel room on March 10.

A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews before returning to search for the missing plane Sunday, March 9, in the Gulf of Thailand.

Members of the Fo Guang Shan rescue team offer a special prayer March 9 at Kuala Lumpur International Airport.

A handout picture provided by the Malaysian Maritime Enforcement Agency shows personnel checking a radar screen during search-and-rescue operations March 9.

Italian tourist Luigi Maraldi, who reported his passport stolen in August, shows his current passport during a news conference at a police station in Phuket island, Thailand, on March 9. Two passengers on the missing Malaysia Airlines flight were reportedly traveling on stolen passports belonging to Maraldi and an Austrian citizen whose papers were stolen two years ago.

Hugh Dunleavy, commercial director of Malaysia Airlines, speaks to journalists March 9 at a Beijing hotel where relatives and friends of the missing flight's passengers are staying.

Vietnamese air force crew stand in front of a plane at Tan Son Nhat airport in Ho Chi Minh City on March 9 before heading out to the area between Vietnam and Malaysia where the airliner vanished.

Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9.

The Chinese navy warship Jinggangshan prepares to leave Zhanjiang Port early on March 9 to assist in search-and-rescue operations for the missing Malaysia Airlines flight. The Jinggangshan, an amphibious landing ship, is loaded with lifesaving equipment, underwater detection devices and supplies of oil, water and food.

Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9. The vessel is carrying 12 divers and will rendezvous with another rescue vessel on its way to the area where contact was lost with Malaysia Airlines Flight 370.

The rescue vessel sets out from Sanya in the South China Sea.

A family member of missing passengers is mobbed by journalists at Kuala Lumpur International Airport on Saturday, March 8.

A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported March 8. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10.

Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8.

Malaysia Airlines official Joshua Law Kok Hwa, center, speaks to reporters in Beijing on March 8.

A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8.

Wang Yue, director of marketing of Malaysia Airlines in China, reads a company statement during a news conference at the Metro Park Lido Hotel in Beijing on March 8.

Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8.

A woman asks a staff member at the Beijing airport for more information on the missing flight.

A Malaysian man who says he has relatives on board the missing plane talks to journalists at the Beijing airport on March 8.

Passengers walk past a Malaysia Airlines sign on March 8 at Kuala Lumpur International Airport.

Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference on March 8 at a hotel in Sepang. "We deeply regret that we have lost all contacts" with the jet, he said.

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

The search for Malaysia Airlines Flight 370

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Photos: The search for Malaysia Airlines Flight 370

"Surely, they must find the plane. That's all I hope for," said Gurusamy Subramaniam, whose son, Puspanathan, is among the missing. "The whole world is out looking for it."

And suggestions about what might have happened to the Boeing 777-200ER continue to multiply, drawing pleas from Malaysian officials to put an end to the theorizing.

"This is why we press on everybody not to speculate," said Azharuddin Abdul Rahman, Malaysia's civil aviation chief. "Because when we have to look into speculative reports, it will distract us from our immediate concern, which is to find the aircraft."

Here are the latest developments in the search and investigation:

Flight evaded radar?

On Monday, the Malaysian newspaper New Straits Times reported that the plane may have flown low to the ground -- 5,000 feet or less -- and used mountainous terrain as cover to evade radar detection. The newspaper cited unnamed sources for its reporting, which CNN could not immediately confirm.

However, Malaysian officials said Monday that they were not aware of the report.

"It does not come from us," said Malaysia Airlines CEO Ahmad Jauhari Yahya.

Analysts interviewed by CNN said that it would be extremely difficult to fly such a large aircraft so close to the ground over a long period of time and that it's not even clear that doing so would keep the plane off radar scopes.

"Five thousand isn't really low enough to evade the radar, and that's kind of where general aviation flies all the time anyway, and we're visible to radar," said former FAA official Mary Schiavo.

READ: Get up to speed

Timeline clarification

Families wait for word of missing flight

Homes of pilot, co-pilot searched

Tracking Malaysia Air flight 370

Yahya said Monday that it wasn't clear whether the final words from the cockpit came before or after the plane's data-reporting system was shut down. Earlier, Malaysian authorities had said the message "All right, good night" came after the system had been disabled.

The voice message came at 1:19 a.m. Saturday, Yahya said. The data system sent its last transmission at 1:07 a.m. and was shut down sometime between then and 1:37 a.m., Yahya said.

READ: Government uncomfortable in spotlight over missing plane

Indian radar

A senior Indian military official told CNN on Monday that military radar near the Andaman and Nicobar Islands isn't as closely watched as other radar systems. That leaves open the possibility that Indian radar systems may not have picked up the airplane at the time of its last known Malaysian radar contact, near the tiny island of Palau Perak in the Straits of Malacca.

Earlier speculation had centered on the idea the plane may have flown over, or perhaps even landed in, the islands, but no trace of it has been found in the region despite intensive searches by Indian military officials.

READ: Pilot: Was that Boeing 777 diverted deliberately? Not necessarily

Chinese response

China said Monday that it had deployed 10 ships, 21 satellites and multiple aircraft to aid in the search. Premier Li Keqiang spoke with Malaysian Prime Minister Najib Razak to ask for more information to help speed the search along, according to a statement posted on the Chinese government website.

READ: Malaysia Flight 370: The 10 big questions

READ: Why are we so gripped by missing Malaysia Airlines Flight 370?

READ: Relatives cling to hope