The 24-year-old Wal-Mart security guard who purchased the military-style rifles used by the San Bernardino shooters -- Enrique Marquez Jr. -- has come under questioning over his relationship with his friend, the male gunman, Syed Rizwan Farook.
Marquez has been cooperating with the investigation since the weekend and has been interviewed a number of times, federal officials said.
Why is Marquez being questioned?
Marquez purchased the two semi-automatic weapons used by Farook, 28, and his wife, Tashfeen Malik, 29, in the massacre that killed 14 people last week.
Marquez purchased a Smith & Wesson M&P15 and a .223-caliber DPMS Model A-15 in 2011 or 2012, around the time Farook is believed to have begun considering carrying out a terrorist attack in the U.S., according to a federal government official who also spoke on condition of anonymity because the investigation is ongoing.
A document by the FBI that lists items seized during a search of the assailants' Redlands home included an "empty long gun box, black, in name of Enrique Marquez."
What is Marquez's relationship with Farook?
Farook and Marquez were next-door neighbors on Tomlinson Avenue in Riverside until a few months ago. Farook and Marquez were good friends, neighbors said. Earlier this year, Farook moved to a rented townhouse in Redlands with his wife.
What other relationship did they have?
The men were tied through a second relationship – Marquez's marriage. Marquez married the sister of the gunman's sister-in-law.
Here are the details:
The gunman's older brother, Syed Raheel Farook, 30, who served in the U.S. Navy between 2003 and 2007, married Tatiana Gigliotti, 30, a Russian woman visiting the United States on a J-1 visa, in 2011, according to a review of a marriage license in Riverside County and a federal official who spoke on condition of anonymity.
The J-1 visa allows foreigners to enter for work-study cultural-exchange programs.
Tatiana Farook has a younger sister, Mariya Chernykh, 25. She also entered the United States on a J-1 visa, a source said. Chernykh married Marquez about a year ago, in November 2014, according to a marriage certificate on file in Riverside County.
The circumstances of the marriage are now under investigation, according to a federal official who spoke on condition of anonymity because the investigation is ongoing.
What was posted on Marquez's Facebook page hours after the deadly attacks?
This cryptic message: "I'm. Very sorry sguys," it read. "It was a pleasure."
What did Marquez do after the shooting?
He checked himself into a mental health facility. He was later interviewed by federal agents, who also searched his home and seized several items.
Where did Marquez live?
Marquez lived at his parents' home in Riverside, a friend said.
What is Marquez's religion?
Marquez converted to Islam around the time Farook began to consider an attack, according to a federal government official who spoke on condition of anonymity because the investigation is ongoing.
Members of the Islamic center where Marquez sometimes attended prayer, however, said his presence at the mosque was uncommon.
Marquez attended the Islamic Society of Corona-Norco four or five years ago, said Yousuf Bhaghani, president of the facility's board of directors.
Bhaghani said he had no recollection of Marquez, but other members of the mosque remembered him.
"We have members who actually are recognized in writing and everything and then you have people who come and go," Bhaghani said. "Enrique was one of those guys who used to come but he was not a permanent member."
Those who do remember Marquez described him as "a very decent person," Bhaghani said.
"There was nothing that seemed like it would sound any alarms or raise any flags, but that was five years ago," Bhaghani said.
"We're trying to figure out how he got converted. Who converted him?" he asked. "Because obviously he's Hispanic and converted Muslim. So far, we really don't have much to go with because the people who remember him remember that he used to come and pray but that's pretty much it."
What did a neighbor of the gunman's brother say about Marquez?
Brittani Adams, 24, said she saw Marquez at the home of Syed Raheel and Tatiana Farook in Corona, where they moved in February.
"He dressed real nerdy: the khakis, the button-up shirt," Adams said. "Those big, god-awful glasses."
Adams said she didn't know Marquez was married -- he and his wife never acted like a couple. She said she never saw them together at the home.
"He would never leave with her, come with her, not hug her," she said. "None of them seemed like they were married. It was very weird."
Tatiana Farook once introduced her sister to Adams, but Adams said the woman spoke little English and kept to herself. Adams and her husband assumed the woman lived there because they never saw anyone drop her off there.
What does a friend of Marquez say about him?
Viviana Ramirez, 23, a friend of Marquez and a fellow student at Riverside City College, described him as shy but said they bonded over their mutual desire to enlist in the military. Marquez, she said, rarely spoke about his family or his marriage but could become playful, even silly, once he dropped his guard.
"He has a really nice smile, he's really welcoming," she said. "He'll play around if he feels very welcome with you."
Marquez never spoke of Farook, according to Ramirez. He talked most often about wanting to join the Navy. He was intensely focused on that goal and physical fitness, choosing to ride his bicycle to and from work and classes at Riverside, she said.
But Marquez's ambition was accompanied by occasional admissions of sadness or loneliness, according to Ramirez. On one of the few occasions that he opened up about his personal life, Ramirez remembers Marquez confessing that he and his new wife were "not clicking."
Marquez's marriage could strike some people as odd, Ramirez said, although she did not find it unusual. Marquez did not live with his wife, she said, and he never explained his living arrangements.
"He never really talked about the friends he had. He wouldn't bring anybody up," Ramirez said. "He was more of an in-the-moment person."
He would sometimes publish melancholy or depressing posts on Facebook, Ramirez said, even though his page was otherwise littered with pictures of him smiling or making clownish faces while wearing a bicycle helmet.
Despite Marquez's links to the shooters, Ramirez said she doubts he would have done anything to help Farook and Malik if he knew what they intended to do with the weapons.
Ramirez is hosting a Christmas party this weekend and said she would gladly welcome Marquez into her home.
"He's never done anything mean. A lot of newspapers call me and want me to talk bad about him," she said. "He is a really good person."
What does a neighbor of Marquez say about him?
Rosie Aguirre, who lives across the street from the Marquez home in Riverside, described her neighbor as a "happy-go-lucky person." Marquez was trying to lose weight, she said, which led to his decision to ride his bike and exercise more.
"He's a real nice guy," she said. "A real normal person."
Aguirre said she was surprised to learn that Marquez had converted to Islam, saying there were never any indications that he was Muslim. She was just as surprised, she said, to learn he had a wife.
"I was shocked. I didn't know he was married," she said. "I've never seen a woman here."
Aguirre said she never heard Marquez or Farook talk about guns or saw any weapons around Marquez's home. The men would just tinker with cars in the driveway for hours, she said.
Where did Marquez go to school?
A spokesman for the Riverside Community College District said that Marquez began attending classes at Riverside City College in the fall of 2009. Marquez withdrew after the 2011 winter term, around the same time he purchased the weapons used by Farook.
The spokesman would not provide further details regarding Marquez's studies.
What does Wal-Mart say about Marquez?
A Wal-Mart spokesman said Wednesday that Marquez had worked for the retail giant since May, but that the company has made the decision to fire him. He did not elaborate.