WHITE PLAINS — If some mental demon drove Steven Lessard to strangle his wife and daughter, it may have been rising to the surface a week earlier, when he got to work at the Indian Point nuclear power plant.

He told his colleagues he’d had a flat tire, but they found his tone alarming.

“He was inordinately concerned about getting the car fixed,” a spokesman for Indian Point owner Entergy Nuclear Northeast, Jim Steets, said. “He was clearly having difficulty with relatively minor issues. His co-workers observed that he was overly stressed and just behaving somewhat irrationally.”

Nuclear plant workers are trained to spot aberrant behavior and required to report it, Mr. Steets said, and Lessard’s coworkers spoke to a supervisor who sent him to the fitness-for-duty nurse at the plant. Lessard, 51, accepted a leave of absence and went home to Lake Peekskill, about 40 miles north of New York City, where the family had a three-story contemporary-style house with a pool and swing set on Maple Road.

“They thought it would be good if he could just take some time off to deal with what was bothering him,” Mr. Steets said.

Lessard’s paid, open-ended leave of absence seemed to start well. His wife had called Indian Point to say thanks and to report he was doing better, Mr. Steets said. Coworkers sent him a fruit basket. He had an appointment with a psychologist or psychiatrist, although Mr. Steets said it’s not known if he kept it. A follow-up conference call with Lessard, his supervisor and the nurse had been scheduled for this week.

On Friday, Lessard’s daughter, Linda, an eighth-grader at Putnam Valley Middle School, went to class as usual, but his wife, Kathy, didn’t show up for her clerk job at Lowe’s. All weekend, relatives were unable to reach the family by phone, and on Monday, police broke in and found the bodies.

Lessard had strangled his wife, 48, and his daughter, 14, in their bedrooms, state police Capt. Keith Corlett said yesterday. Lessard was found dead on a staircase after stabbing himself in the groin with a steak knife, Corlett told news organizations. Autopsies for the rest of the family were scheduled for later yesterday.

There was no forced entry or robbery, Mr. Corlett said. No gun or suicide note was found. A pet cat was rescued.

The police found e-mail evidence suggesting marital trouble, Mr. Corlett said.

Lessard, an engineer, had worked on various projects at Indian Point but was not involved in the operation of the reactor, Mr. Steets said. He had worked there since 1995, and a psychiatric test was part of his screening, Mr. Steets said, but it was his last one.

Only a “critical group” of nuclear plant employees — including security officers and plant operators — get tested again for psychological problems, a spokesman for the Nuclear Regulatory Commission, Neil Sheehan, said. Workers like Lessard, who have “unescorted access” to protected areas, get ongoing behavioral observations and drug-and-alcohol testing under NRC regulations, Mr. Sheehan said.

Mr. Steets said Lessard’s job was “relatively low-stress” and he had earned good evaluations, being called a “valuable contributor.”

“He was well-regarded by supervisors and co-workers but was extremely hard on himself,” Mr. Steets said. “When his supervisor offered to lighten his workload, his reaction was concern about co-workers having to take up his workload.”

Mr. Steets said all of Lessard’s work at the nuclear plant would be checked, but he expected to find no problems.

Debbie Weeks-Petranchik, vice president of the middle school PTA, said she met the Lessards several years ago, when Linda was a Girl Scout.

“The mother was a sweetheart. She always did things for the kids at school,” she said. “He was very quiet and polite. There was nothing you would have said, ‘Hmm.’ They were just an average family.”

The Lessards had maintained a family Internet site, last updated in 2005. On the site, Linda Lessard displayed some “travel adventures,” Kathy Lessard showed off her handicrafts, and Steven Lessard listed his “best beer” (Saranac) and “best ballpark” (Wrigley Field).

An earlier resident of the Lessards’ house also was strangled, the Journal News reported. Olivia Moshier, 21, and another woman were found strangled along a South Carolina highway in 1994; two former Peekskill men were convicted of their murders and sentenced to life in prison.