CORONA, Calif. (Reuters) - In the days since his high school friend and former neighbor Syed Riswan Farook carried out a deadly attack in Southern California, Enrique Marquez has been thrust to the center of a massive federal terror investigation.

A U.S. flag flies at half-mast at a downtown building as people hold candles at a vigil in San Bernardino, California December 7, 2015. REUTERS/Mike Blake -

Until then, Marquez, 24, had appeared to live a mostly unremarkable life, working short stints at a variety of low-paying jobs and residing mostly with his mother and step-father in the suburban Riverside house in which he grew up.

His neighbors there said they were shocked this week when authorities revealed that Marquez had supplied the guns used by Farook and his wife, Tashfeen Malik, in their shooting spree at a holiday party for San Bernardino County employees that left 14 dead and 21 injured.

The relationship between Marquez and Farook dated back to their teenage days, when they were next-door neighbors. Although Farook was four years older, the two boys shared a common interest in automotive repair, and could often be seen working under the hoods of cars on their driveways, friends and neighbors said.

In about 2008, as Farook was becoming more devoted to his Muslim faith, Marquez converted to Islam and began attending Friday prayers at a Corona mosque.

Marquez never seemed terribly pious, however, said Azmi Hasan, manager at the Islamic Society of Corona-Norco. The young convert attended prayers sporadically, sometimes missing them for months at a time, and he confided to Hasan that he was struggling with Islam.

He may have been struggling with something else as well. Marquez told federal investigators in recent days that he and Farook had planned an attack of their own in 2012, but abandoned it, a federal law enforcement source told Reuters.

At around that same time, neighbors report, Marquez and Farook seemed to become estranged. One resident who declined to give her name said she was surprised to see them both standing on the street one day without acknowledging one another. After that, she said, she did not see them together again.

“ISLAM WAS NOT FOR HIM”

At the mosque, Hasan said, Marquez told him about three years ago “that Islam was not for him. He said that he was thinking about becoming a Buddhist.”

Since then, Marquez has moved frequently from one low-paying job to another, seldom leaving much impression on his bosses and co-workers.

“It was hard to make a conclusion about what he was like,” said Sid Hashemi, director of operations at Power Security Group, a small firm based in Corona that hired Marquez part time for a few months in 2013 to help with the company website.

“He just floated around,” he said.

After leaving Power Security, Marquez was hired by United Security Services in Corona, where he made about $9 an hour as an unarmed security guard, mostly working nights on patrol at construction sites or warehouses.

Things started out well for Marquez at the firm, according to the man who hired him, and after two months he was put in charge of the work schedule for about 15 other guards. But Marquez struggled with the responsibility: he forgot when other guards were on vacation; shifts were missed.

“He couldn’t handle the step up,” said the man, an account manager at United Security who asked not to be named. “He just couldn’t take the stress.”

Three months after starting on July 11, 2013, Marquez was gone, company records show.

He subsequently worked security at a local bar. Then, up until the time of the shooting, he worked at a Walmart Supercenter in Corona checking receipts of departing customers. At Walmart, fellow employee Ashlee Sims, 25, described him as somewhat withdrawn and “awkward.”

Outside of work, friends and acquaintances say, he wanted to belong. During a stint at community college, he sometimes met with fellow students for coffee or dinner, and he tried to be entertaining.

“He was quiet, he wouldn’t talk much - and then he would crack jokes” said Viviana Ramirez “He was awesome.”

And although he may have become more distant from Farook, he remained in touch with others in his friend’s family. In 2014, according to state records, Marquez married a Russian woman who was the sister of Farook’s older brother’s wife. The brother, Syed Raheel Farook, was one of the witnesses to the marriage.

Several neighbors say Marquez continued to live at home, and two said they had never seen him with a woman and were surprised to hear he was married.

“I saw no sign of him having a wife - it was only his mom in the house,” said Lori Aguirre, who lives across the street and said she knew Marquez because he sometimes drove her son to and from school.

“I only saw him going to work or coming home alone. It would be a big surprise if he was married,” she said.

Ramirez expressed concern about her friend amid numerous media reports that he checked himself into a mental hospital the day after the attacks. The last communication she had from him came in the form of a cryptic posting to Facebook at 1:45 a.m. that morning:

“I’m. Very sorry guys (sic). It was a pleasure.”