San Bernardino shooting: 'What was motivation?'

(CNN) -- They left no manifesto. The hard drive was gone from their computer. And their new cell phones were apparently smashed.

Syed Rizwan Farook and his wife Tashfeen Malik did their best to fly under the radar. Neither got into trouble with the law. And neither one was on any list of potentially radicalized people.

So, a day after one of the deadliest mass shooting in the United States, authorities are still left wondering what motivated the couple to mow down dozens of people at a holiday luncheon in San Bernardino, California. Fourteen people died and 21 were injured.

Officials say part of the reason may have been Farook's apparent radicalization. Part of it may have stemmed from workplace tensions.

But nothing concrete has surfaced.

"It would be irresponsible and premature for me to call this terrorism," said FBI official David Bodwich on Thursday. "The FBI defines terrorism very specifically, and that is the big question for us: What is the motivation for this?"

Was he radicalized?

One avenue investigators are exploring are Farook's communications with at least one person who was being investigated for possible terror connections. Some were by phone, some conducted on social media.

"These appear to be soft connections," an official said, meaning they were not frequent contacts. Farook's last communication with the contacts was months ago.

The FBI wants to interview some of them to learn more about their conversations with Farook.

A federal official said Farook has "overseas communications and associations" but it's not yet clear how relevant they are to the shootings. "We don't know yet what they mean," the official said.

There are no clear ties to overseas terrorists groups so far.

Were his overseas trips of concern?

Farook had traveled to Saudi Arabia and to Pakistan. But should that raise any suspicions? Islam requires believers to go on a pilgrimage to Mecca in Saudi Arabia once in their lifetime.

His trip to the Hajj raised no red flags, two officials said.

While there, he met Malik, whom he later brought to the United States on a fiancee visa.

Farook is known to have taken at least one more trip to Saudi Arabia, which lasted nine days.

Did that trip help offer a motive? Officials don't yet know.

Was it a workplace dispute?

Could Farook's decision to attack the luncheon have stemmed from a religious dispute with a coworker? One of them, Nicholas Thalasinos, liked to discuss religion and politics.

He was one of the people killed in the Wednesday attack at Inland Regional Center.

Farook and Thalasinos, reportedly a devout Messianic Jew, once had a "heated, passionate" discussion, said Kuuleme Stephens, a friend of Thalasinos, who had called him at work.

The men were sticking by their strongly held positions but were not fighting, Stephens said.

Thalasinos' widow, Jennifer, said he was very verbal about terrorism. "He's very upset about what ISIS has been doing and the radicalized Muslims," she said.

Covering tracks

Whatever the motive, investigators' work to uncover it has been made difficult because the couple appeared to have done a good job covering their tracks.

Two smashed cellphones, both relatively new, were recovered from a garbage can near one of the crime scenes, law enforcement officials said.

A computer found at the shooters' home was missing a hard drive. Investigators suspect it was removed and perhaps destroyed.

Investigators have issued subpoenas to major email and other service providers to try to retrieve the couple's communications.

Keeping quiet

Even those who knew the couple said they didn't raise any red flags.

Doyle Miller, Farook's landlord, said he had "no cause for concern" when he rented out a townhouse to him in Redlands.

"He had no red flags whatsoever; everything checked out," Miller said. "He had good credit reports ... everything."

A religious leader who knew Farook described him as quiet.

"He's a little bit shy, a little bit withdrawn. He doesn't mix with people easily," said Mustafa Kuko, who's the director of the Islamic Center of Riverside where Farook was a regular.

Even his family appeared flabbergasted over the attack.

"I have no idea why he would he do something like this," said Farook's brother-in-law Farhan Khan. "I have absolutely no idea. I am in shock myself," Khan said. "I don't have words to express how sad and how devastated I am."

Heartless killing

But the meek impression belied the heartless act Farook carried out with Malik.First, at the luncheon, armed with .223-caliber long guns and with pistols, they fired 65 to 75 rounds, almost as many as there were people at the luncheon.

Later in the shootout with police that killed them, they fired about the same number of rounds at officers. Two were injured in the gun battle.

In Farook's home, police found thousands of rounds of ammunition, 12 pipe bombs and hundreds of tools to make more explosives with.

With that arsenal, police wondered if Farook and Malik were planning to kill many more and whether the luncheon was their original target. They say what they've found shows there was clear premeditation and planning.

Victims' names released

The San Bernardino County coroner on Thursday released the names of the 14 people killed:

• Robert Adams, 40

• Isaac Amanios, 60

• Bennetta Bet-Badal, 46

• Harry Bowman, 46

• Sierra Clayborn, 27

• Juan Espinoza, 50

• Aurora Godoy, 26

• Shannon Johnson, 45

• Larry Kaufman, 42

• Damian Meins, 58

• Tin Nguyen, 31

• Nicholas Thalasinos, 52

• Michael Wetzel, 37

• Yvette Velasco, 27

Chief: Shooter left party under 'angry' circumstances

The nightmare began at Wednesday's holiday party, which San Bernardino's police chief said that Farook had left abruptly "under circumstances that were described as angry."

He and his wife returned around 11 a.m. (2 p.m. ET) dressed in "black ... tactical gear" and heavily armed. Each had a semiautomatic rifle and a pistol. Then they opened fire, unloading 65 to 75 rounds.

Farook and his wife slipped out a door and into a SUV, which they'd rented locally and had Utah plates.

By then, officers had arrived and began chasing a number of leads. One of them led them to Farook, after another person at the party at Inland Regional Center expressed "some concern over his behavior," Burguan said Thursday.

Officers went with a search warrant to the couple's rented apartment in the neighboring city of Redlands, where they saw a black SUV drive by them, slowly at first, before speeding away.

A police car took up pursuit, and the SUV raced back toward San Bernardino. Then came shots from that vehicle, and a barrage of police gunfire in return.

The couple ended firing at least 76 rounds, while the 21 responding police officers unleashed about 380 rounds of their own, according to Burguan.

By the time the last shot rang out, the SUV was riddled with bullet holes. And Farook and Malik were inside, dead.

Guns and explosives discovered

The husband and wife didn't leave a note to explain their attack, nor did they say anything during the bloodbath, said San Bernardino's police chief.

But they left behind plenty of ammunition and explosives, including three rudimentary devices packed with black powder and rigged to a remote-controlled toy car at the shooting site. None of these went off. The remote was found inside the SUV along with another pipe-like device, which was not an explosive, Burguan said.

Hundreds of unspent rounds were discovered in the vehicle, plus two .223-caliber rifles and two pistols. All were legally purchased three to four years ago, according to Burguan.

More ammunition and more bombs were found inside the couple's rented apartment in Redlands.

Farook and Malik had enough firepower to do more killing.

"They were equipped," the San Bernardino police chief said. "... And they could have done another attack."

Brother-in-law 'in shock'

Farook, an American citizen, was an environmental health specialist with the San Bernardino County health department, which was holding the holiday party. He had worked there for five years.

In an online profile, he described himself as a "Muslim Male living in USA/California/riverside" and his family as "religious but modern."

He "enjoys working on vintage and modern cars, reads religious books, enjoys eating out sometimes. Enjoys travelling and just hanging out in the back yard doing target practice with his younger sister and friends," his profile read.

Farhan Khan said he last talked to Farook, his brother-in-law, a week ago. Farook's family -- including his mother, with whom he and his wife had left their 6-month-old daughter and claimed they were heading to a doctor's appointment -- had tried to reach him all day Wednesday but could not.

"I have no idea why he would he do something like this. I have absolutely no idea. I am in shock myself," Khan said. "I don't have words to express how sad and how devastated I am."

Obama: Mass shootings 'too easy' in U.S.

Whatever their intentions, Farook and Malik caused an avalanche of pain. San Bernardino Mayor Carey Davis saw this suffering in the eyes of relatives of those killed, whose bodies were still in the Inland Regional Center early Thursday, according to county sheriff's spokesman Deon Filer.

Davis told CNN, "The desperation and despair that they feel, we feel that for them also."

At least 10 people were still hospitalized Thursday morning, split evenly between Arrowhead Regional Medical Center and Loma Linda University Medical Center. The Loma Linda hospital CEO, Kerry Heinrich, said two of the victims there were in critical condition.

Once again after a mass shooting, Obama appealed Thursday for something to be done to prevent more heartache.

"Right now, it's too easy," he said. "We're going to have to search ourselves as a society ... to take basic steps that would make it harder -- not impossible, but harder -- to let individuals get access to weapons."