Late Friday, a UPS driver noticed that a package being delivered was addressed to Farook's townhouse and turned around to return it to a distribution facility, San Bernardino Police Chief Jarrod Burguan said in a tweet. The package was held in isolation, and the sheriff's office deployed bomb technicians to inspect the package out of an abundance of caution.

[Previous story, published Friday, 10:06 p.m. ET]

(CNN) -- The San Bernardino, California, mass shooting is now being investigated as "an act of terrorism," FBI official David Bowdich said Friday.

Bowdich said the FBI "uncovered evidence ... of extreme planning."

He said some phone conversations between at least one of the shooters and other people are being investigated. The announcement means the FBI has become the lead investigative agency, taking over from San Bernardino police.

"As of today, based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism," said Bowdich, assistant director of the FBI office in Los Angeles.

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The announcement came several hours after a revelation about Tashfeen Malik, the female shooter in the Wednesday massacre that left 14 dead and 21 wounded.

She posted a pledge of allegiance to ISIS leader Abu Bakr al-Baghdadi on Facebook while the shooting was happening, three U.S. officials familiar with the investigation told CNN.

Malik's post was made on an account with a different name, one U.S. official said. The officials did not explain how they knew Malik made the post.

A Facebook official told CNN that a post that went up about 11 a.m. Wednesday that violated the company's community standards that prohibit the promotion of terrorism or the glorification of violence. It was taken down Thursday.

The official said he couldn't go into details about the nature of the post. The official said Facebook is cooperating with law enforcement.

The mass shooting may have been inspired by ISIS, a law enforcement official said, but none of the officials said ISIS directed or ordered the attack. ISIS has called for people worldwide to launch attacks in its name but isn't known to have claimed credit for what happened in San Bernardino.

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Malik and her husband, Syed Rizwan Farook, burst into a holiday party for the environmental health department he worked for and opened fire. They were killed elsewhere in a shootout with police.

No one has been arrested in connection with the massacre, but authorities say arrests are possible.

"This is looking more and more like self-radicalization," a law enforcement official said.

Farook's relatives didn't know the couple held radical views and have no idea why the couple burst into the party for Farook's co-workers and viciously opened fire, family lawyers said. Nor did the family know the couple had a makeshift bomb lab in the apartment they shared with their 6-month-old daughter and Farook's mother.

"It just doesn't make sense for these two to be able to act like some kind of Bonnie and Clyde or something," Farook family attorney David S. Chesley told CNN's Chris Cuomo. "It's just ridiculous. It doesn't add up."

Another official said authorities haven't ruled out that others may have influenced this radical view. In addition, the law enforcement source said investigators think a workplace issue with religion may have sparked the killings.

Officials: Farook talked to person investigated for terror

Neither Farook nor his wife had gotten into trouble with the law, according to police. Neither was on any list of potentially radicalized people.

But it's not known what connections Malik, who was born and raised in Pakistan and moved to Saudi Arabia about the age of 19, had with any terrorists or groups.

Investigators are exploring Farook's communications with at least one person who was being investigated for possible terror connections. Some were by phone, some 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.

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.

Family lawyer Mohammad Abuershaid said that Farook traveled twice to Saudi Arabia -- first in 2013 for the Hajj, the annual pilgrimage to Mecca that Muslims are required to make at least once in their lifetimes, then again to marry Malik, whom he'd met through an online dating service.

Bowdich said that he went to Pakistan, as well, though Abuershaid denied that.

Did a workplace dispute boil over?

The shooters didn't make it easy for authorities: The hard drive from their computer is gone, and two relatively new cell phones were found smashed in a garbage can near one of the crime scenes, law enforcement officials said.

Bowdich called this an attempt to destroy "digital fingerprints."

Photos: San Bernardino shooting Police officers stand guard as they investigate a suspicious vehicle in Redlands, California, on Wednesday, December 2, after a mass shooting in nearby San Bernardino in which 14 people died and 21 were injured. The shooting took place at the Inland Regional Center, where employees with the county health department were attending a holiday event. The two shooters -- Syed Rizwan Farook and his wife, Tashfeen Malik -- were fatally shot in a gun battle with police hours after the initial incident. Farook worked for the county health department. Hide Caption 1 of 29 Photos: San Bernardino shooting A police officer places a marker along East San Bernardino Avenue, near where the shootout occurred. Hide Caption 2 of 29 Photos: San Bernardino shooting Law enforcement officers search a residential area for suspects who fled after the shooting. Hide Caption 3 of 29 Photos: San Bernardino shooting Police search for the suspects in San Bernardino. Hide Caption 4 of 29 Photos: San Bernardino shooting The residential area was not far from where the shooting occurred. Hide Caption 5 of 29 Photos: San Bernardino shooting Law enforcement officers search a neighborhood in San Bernardino. Hide Caption 6 of 29 Photos: San Bernardino shooting A police officer loads his weapon while pursuing suspects. Hide Caption 7 of 29 Photos: San Bernardino shooting A SWAT team mobilizes during the search. Hide Caption 8 of 29 Photos: San Bernardino shooting People pray on the San Bernardino Golf Course, across the street from where the shooting took place. Hide Caption 9 of 29 Photos: San Bernardino shooting Heavily armed law enforcement officers swarmed the area where the shooting occurred. Hide Caption 10 of 29 Photos: San Bernardino shooting People leave a community center after reuniting with friends and family in the aftermath of the shootings. Hide Caption 11 of 29 Photos: San Bernardino shooting A woman is comforted near the scene of the shooting. Hide Caption 12 of 29 Photos: San Bernardino shooting A police helicopter hovers around the Inland Regional Center. Hide Caption 13 of 29 Photos: San Bernardino shooting Luis Gutierrez gets emotional as he talks about his wife who works in the facility and saw a gunman, according to Los Angeles Times photographer Marcus Yam. Hide Caption 14 of 29 Photos: San Bernardino shooting Two women speak with a firefighter at a triage area near the scene. Hide Caption 15 of 29 Photos: San Bernardino shooting Police stand guard outside of the emergency room at the Loma Linda University Medical Center, where some of the victims were being treated. Hide Caption 16 of 29 Photos: San Bernardino shooting Police escort civilians away from the site of the shooting. Hide Caption 17 of 29 Photos: San Bernardino shooting People walk away from the scene. It is unclear how many people were at the facility at the time of the shooting. Hide Caption 18 of 29 Photos: San Bernardino shooting Police in SWAT gear secure the area. Hide Caption 19 of 29 Photos: San Bernardino shooting Two women embrace at a community center where family members were gathering to pick up people from the scene. Hide Caption 20 of 29 Photos: San Bernardino shooting People gather at the San Bernardino Golf Course. Hide Caption 21 of 29 Photos: San Bernardino shooting People talk to police at the golf course. Hide Caption 22 of 29 Photos: San Bernardino shooting A woman is wheeled away on a stretcher. Hide Caption 23 of 29 Photos: San Bernardino shooting People are moved away from the area by bus. Hide Caption 24 of 29 Photos: San Bernardino shooting Police and fire personnel are seen near the site of the shooting. Hide Caption 25 of 29 Photos: San Bernardino shooting A crowd gathers behind the police line. Hide Caption 26 of 29 Photos: San Bernardino shooting Ambulances pull out of a staging area near the Inland Regional Center. Hide Caption 27 of 29 Photos: San Bernardino shooting A SWAT vehicle carries police officers. Hide Caption 28 of 29 Photos: San Bernardino shooting SWAT teams and a bomb squad were working to clear the buildings where the shootings took place. Hide Caption 29 of 29

Authorities haven't ruled out what President Barack Obama called "mixed motives," meaning that Islamic radicalism and workplace grievances both could have driven the attack.

San Bernardino Police Chief Jarrod Burguan had earlier said there were indications Farook appeared angry when he suddenly left the holiday luncheon at Inland Regional Center, only to return heavily armed with his wife.

But the chief on Friday played down that possibility, saying many people at the holiday party said they saw no evidence of an argument.

Farook family lawyers didn't speculate on his reasoning, though Chesley did say that "at some times co-workers have done silly things, like made fun of Syed's beard."

One of Farook's colleagues killed in the attack, Nicholas Thalasinos, liked to discuss religion and politics. Farook and Thalasinos, reportedly a devout Messianic Jew, had a "heated, passionate" discussion about politics and religion a few days earlier, said Kuuleme Stephens, a friend of Thalasinos, who called him at work while Thalasinos was in the middle of the conversation.

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

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

Reporters go inside attackers home

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Even those who knew Farook best didn't see anything to suggest something nefarious, according to family representatives.

That included Farook's mother, who thought nothing of it when the pair had her watch their 6-month-old girl while they went to a doctor's appointment, Abuershaid said. The lawyer didn't know if there ever was a doctor's appointment -- only that the mother initially worried that Farook had gotten shot, not that he was doing the shooting.

Chesley expressed surprise that Malik, whom he described as being "about 90 pounds, so it's unlikely she could even carry a weapon or wear some type of a vest or do any of this," could have been an active participant in the horror.

A mob of reporters was allowed inside Farook and Malik's rented Redlands townhouse Friday, getting a glimpse at how they lived.

Much of it appeared relatively mundane -- from a hanging clock to couches to many baby items like a crib, ExerSaucer and other toys.

But there were also hints of something more, like shattered glass, boarded-up windows and lists of items seized by federal authorities. The FBI always leaves behind such lists after a search.

Authorities searched the house earlier and found pipe bombs, thousands of rounds of ammunition inside the house and more guns.

The FBI inventoried what they found and returned custody of the home to the landlord. The inventory listed parts for guns, about 5,000 rounds of ammunition, tools, gun boxes, holsters, electronics, Christmas lights, a thumb drive, a laptop and a cellphone with a SIM card missing.

Asked whether authorities were concerned about media entering the couple's apartment, Bowdich replied: "Once we turn that location back over to the occupants of that residence or once we board it up, anyone who goes in at that point has nothing to do with us."