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.
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“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.
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.
Andrew Blankstein is an investigative reporter for NBC News. He covers the Western United States, specializing in crime, courts and homeland security.
Alastair Jamieson is a London-based reporter, editor and homepage producer for NBC News.