Cops: Man who killed 6 was seeking ex

• Police say man killed six people at the home of one of his estranged wife's relatives


• Four children and two adults are dead; one victim is in critical condition


• A victim called police, who intercepted the suspect as he approached a second home


• The suspect, Ron Lee Haskell, 34, surrendered after a three-hour standoff

(CNN) -- A man shot and killed four children and two adults Wednesday at a Houston-area home belonging to relatives of his estranged wife, authorities said.

A seventh person, a 15-year-old girl, was critically wounded in the shooting near Spring, Texas, and taken to a local hospital.

Ron Lee Haskell was arrested after a three-hour standoff that followed a short chase with sheriff's deputies. Haskell, 34, is being held on charges of capital murder, Harris County Precinct 4 Constable Ron Hickman said.

Killed were Stephen Stay, 39; Katie Stay, 33; two boys, ages 13 and 4; and two girls, ages 9 and 7, the Harris County Sheriff's Office said.

Police said Haskell entered the home demanding to know the whereabouts of his estranged wife, who was not there.

The house belonged to relatives of Haskell's spouse, and Haskell might have been an uncle by marriage to at least some of the children, but his relationship to everyone in the home wasn't immediately clear, Hickman said.

"I've not personally in 40 years seen a tragedy in one family that's this horrific," he told reporters Thursday morning.

Officials: Suspect was prepared to take more lives

Haskell, apparently posing as a parcel delivery driver late Wednesday afternoon, entered the home of one of his estranged wife's relatives while only the children were there, Hickman said.

Wearing a purple "Fed-Ex-looking shirt," the suspect "gathered up the children" and waited for the two adults to arrive, he said.

Haskell was not the slain children's father, as police had mistakenly asserted earlier, Hickman said.

Haskell used to drive for a service that had a contract with FedEx, but not since January, FedEx spokeswoman Erin Truxal said.

Investigators didn't immediately know what led to the shooting; Hickman described it as a "domestic situation."

Authorities believe the suspect was prepared to take the lives of other relatives who lived nearby.

As the gunman left, one of the victims called police and notified "us of the location that she believed that he may go from there," Hickman said.

"Grandparents were also targets of him," Harris County Sheriff's Sgt. Thomas Gilliland said. "Fortunately, they were brought out of the house before that could happen, (or) we would have had more tragedy."

Hours-long standoff

Haskell approached the second home and saw officers there; he drove away and the officers pursued him, Hickman said.

Police trapped Haskell in a cul-de-sac, and he surrendered after about three hours, having had discussions with negotiators, Hickman and Gilliland said.

During much of the standoff, Haskell held a gun to his head, while deputies surrounded him with their weapons drawn, police said.

Haskell has children with his estranged wife, but those children were not believed to have been in the Spring-area home where the shootings happened, Hickman said. The estranged wife lives out of state, police said.

Haskell is expected to make his first court appearance Friday at 9 a.m., the county clerk's office said.

Spring is about 20 miles north of Houston.

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CNN's Dave Alsup, John Branch and Marlena Baldacci contributed to this report.