(AP) -- With a young wife, infant daughter and government job, Syed Farook appeared to have arrived at a sweet-spot in life. Friends knew the 28-year-old by his quick smile, his devotion to his Muslim religion and earnest talk about cars he would restore.

They didn't know the man authorities say was busy with his wife, Tashfeen Malik, building homemade bombs and stockpiling thousands of rounds of ammunition for a commando-style assault Wednesday on a holiday party of his co-workers that killed 14 and injured 21.

"This was a person who was successful, who had a good job, a good income, a wife and a family. What was he missing in his life?" asked Nizaam Ali, who worshipped with Farook at a mosque in San Bernardino -- the city east of Los Angeles where Farook killed and died.

As authorities identified the deceased and details about Farook's life began to take shape, the question of what motivated the slaughter remained unanswered.

The FBI was investigating the shootings as a potential act of terrorism but reached no firm conclusions Thursday, said a U.S. official briefed on the probe. Separately, a U.S. intelligence official said Farook had been in contact with known Islamic extremists on social media. Both officials spoke on condition of anonymity because they were not authorized to discuss the investigation publicly.

At the same time, law enforcement officials from local police to Attorney General Loretta Lynch cautioned it could have been work-related rage. Or a twisted hybrid of religion and personal vendetta.

Farook had no criminal record and was not under scrutiny by local or federal law enforcement before the attacks.

Authorities say that the couple sprayed as many as 75 rounds into the room before fleeing. They died four hours later and two miles away during a furious gunbattle with police.

The pair had more than 1,600 bullets when they were killed. Police said they also had 12 pipe bombs, tools to make more explosives, and more than 3,000 rounds of ammunition at home.

Police Lt. Mike Madden, one of the first officers to reach the room at the social services center, where Farook's colleagues from San Bernardino County's public health department had gathered, said the carnage was "unspeakable," the scene overwhelming: the smell of gunpowder, the wails of the injured, the blood, fire sprinklers pumping and fire alarms blaring. All in a room with a Christmas tree and decorations on every table.

The dead ranged in age from 26 to 60. Among the 21 injured were two police officers hurt during the manhunt, authorities said. Two of the wounded remained in critical condition Thursday.

Nearly all the dead and wounded were county employees.

They were remembered Thursday night as several thousand mourners gathered at a local ball park for a candlelight and prayer vigil with leaders of several religions paying them tribute. Mayor Carey Davis urged people to "dedicate and commit ourselves to strengthen our families to overcome." The names of the dead were read and participants sang "God Bless America."

Syed Rizwan Farook was born in Chicago on June 14, 1987, to parents born in Pakistan. He was raised in Southern California.

In July 2010, he was hired as a seasonal public employee and served until December of that year, according to a work history supplied by the county. In January 2012, he was rehired as a trainee environmental health specialist before being promoted two years later. Among his job duties was inspecting restaurants.

The soft-spoken Farook was known to pray every day at San Bernardino's Dar Al Uloom Al Islamiyah mosque. That is where Nizaam Ali and his brother Rahemaan Ali met Farook.

The last time Rahemaan Ali saw his friend was three weeks ago, when Farook abruptly stopped coming to pray. Rahemaan Ali said Farook seemed happy and his usual self. Both brothers said they never saw anything to make them think Farook was violent.

They remember when Farook announced that he would be getting married, saying he had met his future wife online and that she was Pakistani. Farook told the brothers that he traveled to Mecca in Saudi Arabia last summer.

They said he was gone about a month before returning to the U.S. with his wife. Malik arrived on a K-1 visa for fiancées and with a Pakistani passport in July 2014, authorities said.

The two were married on Aug. 16, 2014, in nearby Riverside County, according to their marriage license. Both listed their religion as Muslim. The couple had a 6-month-old daughter who they dropped with relatives Wednesday morning before the shooting.

Patrick Baccari, who sat at the same table as Farook at the employee party, recalled he was short on words and inclined to talk about cars, not religion.

However, a friend of a man killed in the rampage said Farook had a heated conversation about Islam two weeks before the attack.

Kuuleme Stephens said she happened to call Nicholas Thalasinos while her friend was talking with Farook at work. She said Thalasinos, a Messianic Jew who was passionately pro-Israel, told her Farook "doesn't agree that Islam is not a peaceful religion." Stephens said Farook replied that Americans don't understand Islam.

Stephens added that Thalasinos did not think their conversations would turn violent.

Farook legally bought two handguns used in the massacre and their two assault rifles were legally bought by someone else federal authorities wanted to question. That person's identity was not released.

A profile on a matchmaking website for South Asians that matched Farook's name, California hometown, county health job and Muslim faith said his interests included target shooting in his backyard. Though the date of the posting was not clear, it listed his age as 22.

Details about Farook's upbringing are sparse.

He grew up in a turbulent home but later graduated from California State University, San Bernardino, with a degree in environmental health sciences in 2010.

Divorce records depicted a home divided by abuse. Farook's mother alleged in 2006 that her husband, also named Syed, attacked her while her children were present, dropped a TV on her and pushed her toward a car, according to records.

Rafia Sultana Farook filed a petition for a domestic violence order of protection on July 3, 2006, against her husband. She said she was forced to move out with three of her children because her husband continually harassed her "verbally and physically," according to the divorce records.

The Associated Press could not immediately reach the father for comment and was unable to corroborate the allegations in the records. No one answered the door at a home in Corona where a neighbor said the father lived.

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Blood reported from Los Angeles and Tucker from Washington. Contributing to this report were AP writers Ken Dilanian in Washington; Gillian Flaccus, Christine Armario and Justin Pritchard in Los Angeles; Holbrook Mohr in Jackson, Mississippi; Garance Burke in San Francisco; and Jason Keyser in Chicago.