San Bernardino, California (CNN) As federal authorities attempt to piece together the circumstances surrounding last week's terrorist attack in San Bernardino , their far-flung investigation has taken them as far away as Pakistan and Saudi Arabia.

But they have also returned, again and again, to a much closer source of clues: The house next door to the boyhood home of killer Syed Rizwan Farook

Their efforts there are focused on a bespectacled former Walmart employee, Enrique Marquez, and his purchase of a pair of rifles used in the attack that claimed 14 lives

Marquez has acknowledged that he bought the two AR-15s for Farook several years ago.He's also told investigators about a 2012 attack plot that he says he and Farook conceived but did not carry out , U.S. officials told CNN.

Marquez told investigators that part of the reason the two abandoned their plans was that around that time, they were spooked by unrelated FBI arrests of four people charged with attempting to travel abroad to carry out jihad.

Investigators are still trying to corroborate information provided by Marquez and haven't verified details of the alleged plot. Officials caution that Marquez's claim of a 2012 attack could turn out to be false and an attempt to deflect his role in helping buy weapons that Farook later used in the San Bernardino shootings last week.

Marquez, 24, has not been charged with any crime and has told investigators he didn't know about the plans for the San Bernardino attack. Since the shootings, he has waived his Miranda rights, cooperated with investigators and provided information, according to the officials.

Marquez could not be reached for comment. No attorney has come forward.

Mother: He is a good person

Marquez's mother, Armida Chacon, told reporters outside her home Thursday she had no knowledge of her son's involvement in events leading up to the shooting. A sobbing Chacon said: "He was a good person. How would I know? I didn't know," adding that she has not been interviewed by investigators.

"He was a good young man. Whatever I asked him to do he would do. He watched over his brothers. He helped me a lot. He was my right hand around the house," she said after requesting that journalists turn off their cameras before she would speak.

"I want to be left in peace. When I am ready, I will sit down and talk to you. My life changed since Wednesday," Chacon said. "My son is a good person. A good person. The rest, I don't know how it happened. I want to see him."

Authorities initially said they did not believe Marquez had any involvement in the San Bernardino shooting plot carried out by Farook and his wife, Tashfeen Malik , law enforcement officials said. But his role is continuing to be evaluated as he cooperates with investigators and they assess his information, officials said.

FBI Assistant Director David Bowdich was tight-lipped when asked about his status in the investigation at a news conference earlier this week. "I'm not prepared to discuss Mr. Marquez at this point," Bowdich said.

Related by marriage

According to county records, Marquez and Farook are related by marriage, and the address on his marriage license is the current address of Farook's father. Marquez was married last year with Farook's brother as a witness.

He converted to Islam several years ago and attended the same mosque as other members of the Farook family.

Marquez, who was a state licensed security guard until his license expired last year, checked himself into a mental health facility in the wake of the attacks, according to law enforcement officials who spoke on the condition of anonymity.

According to officials, Marquez told investigators that he and Farook were on the path to radicalization as early as 2011. That same year, Marquez bought the first of two rifles for Farook.

Marquez gave the rifles to Farook shortly after purchasing them but did not report the transfer of ownership, two law enforcement officials said Tuesday.

Such transactions could be a violation of California law, the officials said.

Home raided, job terminated

Heavily armed FBI agents descended on his home on Tomlinson Avenue in Riverside early Saturday to serve a search warrant, waking neighbors with a bullhorn announcement for the occupants to come to the door. Agents ultimately forced entry through the garage. They returned a day later for a consensual search to retrieve items not covered in the scope of the warrant, according to a law enforcement official.

Agents also visited the Walmart store in Corona where Marquez worked. A spokesman for the retailer said Marquez has worked for Walmart since May, but "the decision has been made to terminate him."

A co-worker who asked not to be named said she was twice interviewed by FBI agents earlier this week. They asked about Marquez's personality and interests, the co-worker said.

In a brief interview with CNN, she said she told investigators she had no knowledge of Marquez using weapons or of having any link to the killers, whom she did not know. She did not associate with Marquez outside of work, she said.

Neighbors and car-work buddies

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Neighbors of the Tomlinson Avenue homes where Marquez and Farook lived next door to one another recalled the two working on cars together but did not know whether their relationship extended beyond that shared interest.

One neighbor, who asked not to be named, said Marquez seemed like a nice young man. "He was a good guy," the neighbor said.

Another neighbor, Freddy Escamilla, said he'd recently run into Marquez on the street and that he was typically subdued, nodding hello but not saying much.

"He never really talked to anyone," Escamilla said, adding that he was "really introverted. Very introverted."

Marquez converted to Islam and attended mosque sermons on and off for a couple years, said Azmi Hasan, who has served as facility manager of the Islamic Society of Corona-Norco since 2000.

Hasan said Marquez acted goofy, describing one instance when he saw him outside the mosque laughing out loud to himself. When approached by Hasan about what was so funny, Marquez said he wasn't laughing.

Hasan said Marquez attended sermons by himself but stopped coming about two years ago. He said he ran into Marquez, who he recalled as quiet and introverted, at a party and asked why he hadn't been coming to sermons more often.

When questioned about why he wasn't coming to sermons, Marquez would say he was busy, according to Hasan. At one point, Hasan said, Marquez responded that Islam was not working for him.

Hasan said Syed Rizwan Farook's sister and brother-in-law also attended the mosque but that he'd never seen Marquez in their company.