The FBI says it is now investigating Wednesday's mass shooting in San Bernardino, Calif., as an "act of terrorism."

"We have uncovered evidence that has led us to learn of extensive planning," said David Bowdich, assistant director of the FBI's Los Angeles office. He would not give further details about why the bureau made the determination, telling a news conference "there's a number of pieces of evidence that has pushed us off the cliff."

Syed Rizwan Farook, 28, and wife Tashfeen Malik, 27, carried out the attack at the Inland Regional Center, where Farook's co-workers had gathered for a holiday luncheon. Fourteen people died and 21 were wounded.

San Bernardino Shootings

Lawyers for the family of Syed Rizwan Farook, 28, said there is no evidence he was affiliated with a terrorist group. (California Department of Motor Vehicles /Associated Press)

The pair were killed about four hours later following a gun battle with police.

Bowdich also confirmed authorities were looking into an apparent Facebook post in which Malik had pledged allegiance to ISIS leader Abu Bakr al-Baghdadi.

Earlier Friday, multiple U.S. media outlets reported Malik had expressed "admiration" for al-Baghdadi under an account that used a different name and then deleted the messages before the attack.

"We're still continuing to look into that," Bowdich said. "I know it was (on) a general (Facebook) timeline, and there was a pledge of allegiance."

Facebook told CBC News it had identified and removed the account on Thursday because it "violated community standards."

Speaking at a separate event, FBI Director James Comey cautioned that the bureau's investigation has not yet shown evidence that the slain shooting suspects were part of a larger group or members of a terror cell.

Comey said that despite growing signs of "radicalization" among the two suspects, there "is a lot evidence that doesn't quite make sense."

He said the suspects did not appear on the FBI's "radar screen" prior to the shooting.

California Shootings, David Bowdich of FBI

David Bowdich, centre, assistant FBI director in charge of the bureau's Los Angeles office, tells reporters on Friday that the San Bernardino mass-shooting is officially being investigated as an 'act of terrorism.' (Jae C. Hong/Associated Press)

San Bernardino shooting

Police officers stand watch as people leave Friday prayers at the Dar Al Uloom Al Islamiyah-Amer mosque in San Bernardino where Farook was seen two to three times a week. (Mike Blake/Reuters)

The disclosure about Malik's online activities provided the first significant details suggesting a motive for her participation in the shooting. Malik's alias on Facebook and specifics about her postings were not publicly disclosed.

There was no sign that anyone affiliated with ISIS communicated back to her, nor were there signs of any operational instructions being conveyed to her, according to reports.

Phones smashed

Malik and Farook also took steps to erase their "digital fingerprints," with investigators finding two crushed cellphones in a garbage can near one of the crime scenes.

A computer found at the couple's home was missing a hard drive, and investigators suspect it may have been removed and destroyed.

'We do hope that the digital fingerprints left by these two individuals will take us toward their motivation.' - David Bowdich, FBI

The FBI told the New York Times that it has found evidence that Farook began deleting information from an electronic device he owned at least a day before the attack.

"We do hope that the digital fingerprints left by these two individuals will take us toward their motivation," Bowdich said. "That evidence is incredibly important."

He also revealed investigators were aware of "telephonic conversations" between the couple and other people of interest in FBI probes. Authorities weren't aware of any overseas communications as of yet.

Attorneys for family San Bernardino shooter speak out1:06

And they have spoken to the person who purchased the weapons found in the shooters' possession, Bowdich said, noting that individual is "not under arrest." Police previously said the guns were legally purchased.

The couple had two assault-style rifles, two semi-automatic handguns, 6,100 rounds of ammunition and 12 pipe bombs in their home or with them when they were killed.

Media allowed inside couple's home

San Bernardino California Shooting inside suspects Redlands house Dec 4 2015

A member of the media films a wall tapestry in the living room of the apartment shared by Farook and Malik after the building landlord invited media in. (Chris Carlson/AP)

Earlier Friday, the landlord of the couple's townhouse rental allowed media inside the property, revealing a scene of busted doors and broken glass but also commonplace items like children's toys and dirty dishes.

Some questioned whether reporters should have been allowed inside the former crime scene, but police said the apartment was released Thursday night.

"Once we turn that location back over to the occupants ... anyone who goes in at that point has nothing to do with us," said Bowdich.

'They didn't tell me they were terrorists when they moved in. That wasn't on the application.' - Doyle Miller, landlord of shooters' home

The townhome's owner, Doyle Miller, 81, said he last saw Farook about a month ago and was last inside the home's garage, where bombs and bomb-making material were found, in May.

He said he rented the unit to the couple because they "seemed like the best applicants."

"They had jobs, a good credit report," Miller told CBC's Matt Kwong. "They didn't tell me they were terrorists when they moved in. That wasn't on the application."

Brother-in-law hoping to adopt daughter

Lawyers for the Farook family said Friday there was no evidence that either shooter had been associated with any terrorist group.

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Mohammad Abuershaid and David Chesley said Farook was an isolated individual with few friends. They said the family described Malik as a "caring, soft-spoken" housewife. She wore a veil that covered her face and didn't drive.

The lawyers also said Farook's mother lived with the couple, helping to care for their six-month-old daughter, but stayed upstairs. The family was aware that Farook owned two handguns.

California Shootings

People gathered at San Manuel Stadium in San Bernardino Thursday evening for a vigil for the victims of Wednesday's shooting. (Mark J. Terrill/Associated Press)

Farhan Khan, who is married to Farook's sister, is beginning the legal process to adopt the child. Khan said Friday he was "very upset and angry" at Farook.

"You left your six-month-old daughter," he told NBC News. "In this life some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind.... What did you achieve?"

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With files from The Associated Press and CBC News