The Muslim husband and wife behind the mass shooting in San Bernardino began erasing their digital footprint a day in advance of the deadly attack, deleting email accounts, disposing of hard drives and smashing their cellphones, according to law enforcement investigators who are treating the probe as a counterterrorism case.

Investigators edged closer Thursday to the conclusion that Syed Rizwan Farook and his wife, Tashfeen Malik, were radicalized by Islamist extremists either in the U.S. or during trips to the Middle East, including Saudi Arabia.

Working to determine whether terrorism was the motive in the attack at a government-organized holiday party, officials scrambled to re-create the travels, contacts and lifestyle of Farook, a 28-year-old U.S. citizen of Pakistani descent, and his 27-year-old Pakistani wife.

Intelligence sources said there is still no explicit evidence linking the two to known terrorists on U.S. or foreign watch lists and that the couple was not on the FBI’s radar before Wednesday’s deadly shooting rampage. But their cache of thousands of rounds of ammunition and explosives found in a home that the couple rented raises major concerns about a wider plot, or a plan to act on their own to hit other targets.

Authorities have tracked down at least four people from the Los Angeles area who were previously under investigation by U.S. counterterrorism officials and were found to have had communication with Farook in the past, said a law enforcement source involved in the investigation.

But details about the individuals were murky. The law enforcement source said the last contact any of the four had with Farook was in June, and asserted that nothing fruitful came from the interviews about Wednesday’s attack.

With that as a backdrop, officials were working Thursday night to determine what motivated Farook and Malik to storm a conference room inside the Inland Regional Center during a holiday party and spray the room with bullets. Fourteen people were killed and another 21 were injured in either the shooting or a police pursuit and gunbattle that occurred hours later.

The San Bernardino County sheriff-coroner’s office continued to work at the scene of the massacre documenting evidence and removing several of the bodies that remained. It was not until Thursday afternoon that officials had contacted the next of kin for all 14 of the victims and that their names were publicly released.

Candlelight vigils were held in their honor Thursday night.

Farook and Malik died in the firefight as they tried to evade police in a rented SUV, firing 76 rounds at officers who unleashed 380 rounds of their own and riddled the vehicle with bullet holes. More than 1,500 rounds of unspent ammunition were found in the vehicle, according to police.

One official, who spoke anonymously with The Washington Times, cautioned against jumping to conclusions. The official said the investigation is still young and asserted that “radicalization can happen in a number of different inflection points and in a number of different ways. It’s not a one-size-fits-all sort of thing.”

But fresh details continued to emerge about the background of the two — supporting the notion that the attack was planned and not a spur-of-the-moment response to a party snub or quarrel.

The couple had a 6-month-old daughter, who was left in a relative’s care before they stormed the Inland Regional Center bedecked in tactical-style clothes and spraying 75 rounds into the conference room.

Three pipe bombs configured as one large explosive device were recovered from the center. Police also found bomb-making materials, 12 pipe bombs and more than 4,500 rounds of ammunition inside the couple’s home.

“This is not your average investigation,” said David Bowdich, assistant director in charge of the FBI’s Los Angeles field office, mentioning the multiple crime scenes, victims and recovered evidence. “It will take time.”

Officials involved in the investigation said the couple appeared to have gone to great lengths to conceal themselves and make difficult investigation of their plans — further suggesting premeditation.

A cellphone recovered from Malik’s body was newly purchased and had been used only recently. Two other cellphones that were recovered had been smashed with a hammer and were expected to be sent to the FBI’s forensic lab in Washington for examination.

Authorities also noted that a hard drive and motherboard were missing from a computer found at the Redlands, California, home that the couple rented.

Farook worked as a health inspector for the San Bernardino County Health Department, the agency holding the holiday party. He had been in attendance at the event but was reported to have left and returned about 30 minutes later with his wife.

Accounts have emerged that Farook was engaged in a dispute with someone at the party before he left, but police at a Thursday press conference said they were still investigating the information and had varying stories from witnesses.

“There appears to be a degree of planning that went into this,” said San Bernardino Police Chief Jarrod Burguan. “Nobody just gets upset at a party and goes home and plans an attack like that.”

The State Department confirmed Thursday that Malik held a Pakistani passport and was in the U.S. on a K-1 visa reserved for fiancees of U.S. citizens. Department spokesman Mark Toner told reporters that Malik was granted the visa in Islamabad and would have had to undergo “counterterrorism screening” to receive the document from U.S. authorities.

While other officials said the visa was issued in 2014, details about international travel by Farook’s and Malik’s travels were not fully clear.

Investigators said Farook traveled at one point to Pakistan before returning to the U.S. in July 2014 with his bride-to-be. Sources also confirmed that Farook went to Saudi Arabia for nine days in summer 2014. It was unclear whether the couple traveled to other countries together.

Mr. Toner told reporters that the State Department does not track the movements of U.S. citizens overseas.

The official who spoke anonymously with The Times said it was entirely possible that the couple had contact with extremists during their foreign travels or while in the U.S., but that the extent of those contacts was unclear.

“If you’re in a minority in whatever country you’re in, a lot of the relationships may be tight-knit, so the chance that you’re going to have contacts with people that may be of interest to authorities, that’s plausible,” the official said. “But the chance that there’s a close relationship there is unknown and won’t be known until further investigation.”

Farook’s brother-in-law, Farhan Khan, said Wednesday that he was shocked by Farook’s actions.

“I have no idea, no idea, why he would do that. Why he would do something like this,” Mr. Khan said.

Two weeks ago, Farook and one of the co-workers he killed, 52-year-old Nicholas Thalasinos, had a heated conversation about Islam, according to Kuuleme Stephens, a friend of the victim’s.

Ms. Stephens said she happened to call Mr. Thalasinos while he was at work and having a discussion with Farook. She said Mr. Thalasinos told her that Farook “doesn’t agree that Islam is not a peaceful religion.”

Other co-workers recalled bits and pieces about the man, whom many viewed as mild-mannered and happy.

Co-worker Patrick Baccari said he was sitting at the same table as Farook during the holiday party when he suddenly disappeared. Mr. Baccari said that when the shooting started, he took refuge in a bathroom and suffered minor wounds from shrapnel slicing through the wall.

The shooting lasted about five minutes, he said, and when he looked in the mirror he realized he was bleeding.

“If I hadn’t been in the bathroom, I’d probably be laying dead on the floor,” he said.

Mr. Baccari described Farook as reserved and said he showed no signs of unusual behavior. Earlier this year, he traveled to Saudi Arabia, was gone for about a month and returned with a wife, and later grew a beard, Mr. Baccari said.

⦁ This article is based in part on wire reports.

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