Grim details emerged Thursday about the massacre at a California holiday party that left 14 people dead and 21 others hurt, but investigators are still trying to answer the key question: Why?

The couple who unleashed the bloodbath Wednesday fired as many as 75 rounds at the Inland Regional Center in San Bernardino and tried to slaughter more with a rigged pipe bomb that never detonated, authorities said Thursday.

Syed Rizwan Farook, 28, who was born in Chicago, and his wife, Tashfeen Malik, 27, were killed in a shootout with police a few miles away more than four hours after the rampage.

The couple — clad in black tactical gear but not bulletproof vests — fired 76 more rifle rounds during a pursuit with police, San Bernardino Police Chief Jarrod Burguan told reporters. Officers returned about 380 rounds.

Authorities said they are still trying to determine why Farook and his wife attacked the Inland Regional Center.

"We do not know the motive. ... It would be irresponsible of me and be way too early to speculate on motive," said David Bowdich, assistant director in charge of the FBI in Los Angeles.

"There was obviously a mission here," Bowdich said. "We do not know why. We do not know if this was the intended target or something triggered him."

A photo of Syed Rizwan Farook California DMV

But authorities told NBC News on Thursday that Farook appeared to have been radicalized. They said he had been in touch with people in the Los Angeles area who have expressed jihadi-oriented views.

Intelligence sources told NBC News that Farook appeared to have been in some form of communication with people overseas who are persons of interest to U.S. authorities.

Farook and Malik had two .223 caliber assault-style rifles — a Smith & Wesson M&P 15 and a DPMS model — that were legally purchased and registered, Burguan said. They also had two 9mm semiautomatic handguns.

They appeared to have tried to modify one of the rifles to be fully automatic, a senior law enforcement source told NBC News on Thursday. The modification failed, the source said.

A fully automatic weapon fires multiple rounds with one pull of the trigger, while each shot from a semiautomatic weapon requires a separate trigger pull.

The second rifle — possessed by Malik — was compliant with California's strict gun laws when it was purchased, but it had been modified to accept high-capacity magazines, the source said. Farook and Malik used what appeared to be multiple 30-round rifle magazines, some of which were taped together.

Syed Rizwan Farook. iMilap.com

In total, the pair carried more than 1,400 .223 caliber rounds and 200 more 9mm rounds during the gunbattle. The three pipe bombs found at the social services center were stitched together on a remote-controlled car, but for whatever reason, it didn't explode, he said.

A search of a home connected with the couple turned up 12 pipe-bomb-type devices and a cache of ammunition, Burguan said — including 2,000 9mm rounds, 2,500 .233 caliber rounds and several hundred .22 Long Rifle rounds.

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Police also confirmed that the couple had rented a black Ford Expedition three or four days before the carnage. The car, which was supposed to have been returned Wednesday, was used as their getaway.

Farook, a San Bernardino County health inspector, initially attended the office Christmas banquet on Wednesday morning, but he left about 15 to 30 minutes before the shooting began around 11 a.m. (2 p.m. ET), officials said. Police received information by someone who was concerned about his behavior at the event.

Farook traveled to Saudi Arabia at least twice, in 2013 and 2014, acquaintances and Saudi officials told NBC News.

In 2013, he went on Umrah, a pilgrimage to Mecca. Nazeem Ali, 23, who attended Dar-Al-Uloom Al-Islamia mosque in Muscoy, a suburb of San Bernardino, with Farook, said Farook told him that he planned to meet the fiancée he had met online — Malik — and to get married at the Black Stone in the Grand Mosque of Mecca.

Malik, who was of Pakistani background, arrived in the United States on a K-1, or fiancée, visa and attained permanent resident status by virtue of their marriage two years ago.

Mustafa Kuko, director of the Islamic Center of Riverside, another mosque Farook attended for several years, told NBC News that Farook and Malik had their wedding reception at the center in 2013.

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Last year, Farook returned to Saudi Arabia, the Saudi Embassy in Washington confirmed Thursday. The embassy said he arrived in the summer and stayed for nine days. It gave no further details.

In May, Farook and Malik had a baby, whom they left with Farook's mother, claiming they they had a doctor's appointment, said Farhan Khan, Farook's brother-in-law.

The grandparents first became worried when they got a call from the media about 2 p.m. asking whether they knew that Farook was a suspect, Khan, who is married to Farook's sister, told NBC News.

"I just cannot express how sad I am for what happened today," Khan said Wednesday night at a news conference held by the Council on American-Islamic Relations, a Muslim advocacy group. "I am in shock that something like this could happen."

Khan said he had spoken with Farook a week ago, adding: "Why would he do something like this?"

A profile under the user name "farooksyed49" on the dating website iMilap.com featured a picture of Farook, which Khan confirmed was his brother-in-law. It said the user was Muslim American born in Chicago.

"Farooksyed49" wrote that he was looking for marriage who lived in "California/riverside" — a community about 11 miles south of San Bernardino. His age is listed as 22. IMilap.com describes itself as a "Site for People with Disabilities and Second Marriage."

The user said he was from a "religious but modern family" and listed "Eastern and Western Mix" under family values.

"Enjoy working on vintage and modern cars, read religious books, enjoy eating out sometimes travel and just hang out in back yard doing target practice with younger sister and friends," according to the profile.

NBC News couldn't immediately confirm the authenticity of the profile.

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San Bernardino Mayor Carey Davis spoke of his "shock" at the events Thursday morning on TODAY.

"The priority has been public safety. Our police department responded as effectively and safely as they could," he said. "We need to stay on a high level of alert, but at the same time, we can't be paralyzed by these instances."

Salihin Kondoker, whose wife was shot during the rampage, told TODAY that his son first learned of her involvement when a friend saw television images of her being treated at a hospital.

Kondoker said his wife was recovering from gunshot wounds to her right arm.

"She's doing remarkably well," he said. "She said bullets were flying all over the place. She corralled herself into the bathroom but realized her hands were bleeding."

He said Wednesday's assault should be categorized as terrorism.

"Any killing of a human being should be an act of terrorism," Kondoker said. "I think we need to control our gun law in this country. No matter what, any killing of any human being is an act of terrorism."