LOS ANGELES — In the years leading up to his death in the locomotive of a commuter train, engineer Robert Sanchez's life was marked by personal tragedy, jail time, and concerns about his health and job security.

His HIV-positive companion had committed suicide, he was concerned about his diabetes and he feared a brush with the law that could end the career he loved.

The National Transportation Safety Board is looking at Sanchez's background after determining human error — and not mechanical or equipment failure — was likely to blame for the collision Friday between Metrolink 111 and a Union Pacific freight train that killed 25 people, including Sanchez, and injured 135 others.

The NTSB is examining the training, efficiency, personnel and medical records of Sanchez to determine why he blew through a red signal and failed to hit the brakes on Metrolink 111.

"We're not just limiting ourselves to the schedule he worked that day," National Transportation Safety Board member Kitty Higgins said. "We walk it back to see whether there was anything else that might be of significance in looking at the whole picture."

In addition to examining what Sanchez did moments before the collision — including text messages sent from his cell phone while on duty that day — investigators plan to look at his background and what he was doing in the days before the accident.

NTSB experts will look at his work, sleep, rest and eating patterns in the three days leading up to the crash, said Ronald Schleede, a former NTSB investigator who once headed a division specializing in human performance in transportation accidents.

That could lead to a deeper investigation and interviews with the engineer's peers and families and look at various records.

"It's not an easy thing to do; you can't put yourself in someone else's head, you can't really put yourself there and think what this person was thinking or see what this person was seeing," he said. "You can only deduce a little bit."

Friends said Sanchez was a friendly man, who was private and quiet unless he was talking about the two things he loved: trains and dogs.

"He was very outgoing when it came to socializing in the environment we all knew him in, which was the dogs," said Michelle Paulin, a professional dog handler who used to take Sanchez's Italian greyhounds to shows.

But he had his share of problems, starting with his guilty plea to misdemeanor grand theft in 2002 for stealing video game consoles from a store.

"One of his biggest concerns was how is this going to impact his career," defense attorney Wilson Wong said of his client's guilty plea. "He thought: 'Could this cause my career to come to an end?"'

Months later, on Feb. 14, 2003, Sanchez's partner, Daniel Burton, hanged himself in the garage of the home they shared in Crestline, a community in the San Bernardino Mountains about 80 miles east of Los Angeles.

Burton's sister, Carolann Peschell, said she suspected foul play and never believed her 39-year-old brother, who was HIV-positive, would have killed himself. He had found a job at a gourmet restaurant and sounded well when she spoke to him two weeks before his death.

"He was doing fine; he was happy, no signs of depression," Peschell said. "We didn't feel my brother was capable of doing this to himself."

Peschell, who described Sanchez as "very odd, very strange," said her suspicions were ruled out by San Bernardino County sheriff's investigators.

A coroner's report said the two men had argued the night before Burton's body was found; Sanchez had told Burton they should break up.

Peschell kept her brother's purported suicide note, which read: "Rob, Happy Valentine's Day. I love you. Please take care of yourself and Ignatia. I love you both very much." Ignatia was their dog.

Neighbors said Sanchez was a recluse, saying little to them during the last year of his life. But friend and fellow dog breeder Lilian Barber remembered Sanchez as well liked but lonely.

"He was always relentlessly upbeat. He always had a smile on his face," Barber, 77, of Murrieta, said Wednesday.

The two became friends in 2004 after she met Sanchez through a dog breeder referral service and he was trying to breed Ignatia. They attended a dog show together in San Francisco, then started going out to lunch together almost once a week.

"The problem with going to lunch with Rob was he would come back and talk to me and my husband for the rest of the day," Barber said. "He was pretty lonely."

Sanchez used to have a train route through San Diego County and moved to Menifee, in southeastern Riverside County, to be closer to work, Barber said. When he transferred to the Ventura County route, he moved to La Crescenta, a section of Los Angeles.

Attempts to reach Sanchez's family have been unsuccessful. Barber, who last spoke with Sanchez eight months ago, said Sanchez told her he grew up in a farming community in Nevada and was involved with the 4-H Club as a child.

In their last conversation, Sanchez told Barber that he was on his way to meet her and wanted to go to lunch, she said. When she replied that she was too busy, he said "that's fine" and abruptly hung up.

"I never realized I probably knew him better than anybody else other than his family. I don't think I knew him that well at all," Barber said. "I've missed him for the last year, to tell you the truth, because I was so used to seeing him regularly."