San Bernardino shooters used four guns, explosive device: ATF

The suspects in the San Bernardino holiday party shooting were armed with four guns, an explosive device and several magazines of ammunition in a "well-planned" attack, police and federal officials said.

Syed Rizwan Farook, 28, a U.S.-born health inspector for San Bernardino County, and Tashfeen Malik, 27, were carrying two .223-caliber assault rifles and two semi-automatic handguns.

Two of the four weapons were found to be legally purchased, sources familiar with the investigation told NBC News.

An "explosive device" was found at the scene of the Wednesday's shooting spree, while items made to look like pipe bombs were thrown during the pursuit, according to Bureau of Tobacco, Firearms and Explosives spokeswoman Meredith Davis.

See photos from the scene in San Bernardino:

"They were in tactical gear and had several mags full of ammunition fashioned to their body so they were ready for a gunfight should that occur," Davis said. "With my experiences with other mass shootings, there are indicators this was well-planned out and it was to be violent in nature."

San Bernardino Police Chief Jarrod Burguan said the two .223-caliber assault rifles were a DPMS model and a Smith & Wesson MMP 15 model, while the two semi-automatic hand guns were manufactured by llama and Smith & Wesson.

"The weapons we recovered — two long guns and two pistols — they will also have serial numbers on them that we can trace as long as they were produced by manufacturers either here in the U.S. or imported to the U.S.," Davis said.

Bomb technicians were sent to make safe the explosive device located at the Inland Regional Center, a state-run center for people with developmental disabilities in San Bernardino, Davis added. The device was found during a sweep of the scene.

She added that fears of pipe bombs were based on "an abundance of caution" after similar tactics were used by the Boston Marathon bombers.

"I do recall in the Boston bombing that they threw devices, explosive devices at the police as they were being pursued," Davis said. "They planned that out very thoroughly, I would make a connection to that."

However, ATF experts found that the items thrown were in fact "a very thick pipe with a piece of cloth that was made to look like a fuse [but] did not contain any explosives."

Burguan echoed her comments that the assault appeared to have been well-planned.

He reporters: "I think that, based upon what we have seen and based upon how they were equipped, there had to be some degree of planning that went into this. I don't think they just ran home, put on these types of tactical clothes, grabbed guns and came back on a spur of the moment."

Davis described the involvement of a woman in the attacks as "odd."

"Being a female that is experienced with firearms and explosives ... in my world it's odd to me ... when we look at cases like this across the U.S. and maybe across the world, it is unusual to have a female suspect," she said.

PHOTOS: California mourns the victims of the San Bernardino shooting

More from NBC News:
San Bernardino Shooting: Suspects Left Baby Daughter With Grandma
Obama Calls for Gun Law Changes After Latest Mass Shooting in California
'Thoughts and Prayers' Backlash After San Bernardino Shooting