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From NBC Bay Area

The FBI has launched a nationwide manhunt for a man named Ryan Kelly Chamberlain II in connection with Sunday's investigation at an apartment building in San Francisco's Russian Hill neighborhood.

FBI agents, San Francisco police and fire crews, and a hazardous materials team swarmed the building between Jackson and Polk streets early Saturday morning in a search that continued into the early hours of Sunday.

FBI spokesperson Peter Lee said at a press conference Sunday that the FBI is looking for Chamberlain, 42, a San Francisco resident, for possession of explosive material at his residence.

Chamberlain is 6' 3" tall, weighs 225 lbs and has blue eyes and brown hair, according to Lee. He was last seen south of San Francisco Sunday, driving a white 2008 Nissan Altima with a California or Texas license plates of either 7FQY085 or BX9M042. He is believed to be wearing a dark blue hooded sweatshirt and blue jeans and is considered armed and dangerous, Lee said.

The FBI is encouraging anyone with any information on Chamberlain to notify the agency at 415-553-7400 or call 911.

The San Francisco FBI office told NBC Bay Area Sunday that the agency was executing a search warrant at Chamberlain's apartment as part of a criminal investigation.

the FBI said the investigation poses "no threat to public safety," and that protective equipment had been used Sunday in the interest of agent safety. Lee said that contrary to what was reported by some news sources, there was no conclusive evidence of the deadly poison ricin at Chamberlain's apartment.

An FBI spokesperson said that a bomb squad had shown up at the scene of the investigation, but that it turned out to be a false radiation alarm.

An FBI source told NBC Bay Area that crews found a chemical in the apartment that, if not handled correctly, can kill. The person may have bought the chemical -- which is often used in assisted suicide situations -- on the internet, according to the source.

The FBI declined further comment on the intent or motive behind the explosive material in light of the ongoing investigation.

"Actually, as I was walking down here, I thought it was a film set," onlooker Faisal Choudry said. "And somebody said the FBI is here investigating, and they’ve been here since the morning."

Police blocked off a section of Jackson Street between Polk Street and Van Ness Avenue Saturday. Jackson street was reopened around 12:30 a.m. and fire and hazmat crews left the scene Sunday morning.

Those who live in the area said it had been an interesting afternoon.

"There are a lot of police, a lot of fire, and there was a bomb squad here earlier," neighbor Dianne Dubois said. "That's very scary for those of us who live here, but it seems like everything is going OK. But it's hard to know. We don't know anything about what's going on, so we have no information at all."

Neighbors said that the FBI had not told them anything about the investigation.

"It seems a bit more chaotic down here than it was inside," neighbor Tiffany Fechtar said. "The person’s door is open. There’s stuff outside. They’ve got the hazmat suits on, and I don’t know. what’s going on."

Those who live inside the building told NBC Bay Area the FBI has taken at least two computers from the apartment.

This undated image provided by the FBI shows Ryan Kelly Chamberlain, II, who is being sought by the FBI, who has issued a nationwide alert to law enforcement agencies about this man they consider armed and dangerous who is wanted for investigation of possessing explosives.FBI via AP

Neighbors said they do not know much about the man who lives in the unit except he is in his 40s and works in marketing. They described him as quiet and polite.

"I know this person who lives in this apartment," Fechtar said. "It’s just kind of weird. He seems like a nice guy, and he probably is."

When reporters asked Lee whether the FBI is using Chamberlain's apparent social media presence to help with the investigation, Lee declined comment or confirmation that the online accounts belonged to Chamberlain.

Chamberlain's warrant and affidavit are currently sealed, Lee said. His apartment is being treated as an active crime scene and currently remains sealed, he said.

Crews wearing hazmat suits were seen going in and out of the building and people gathered on the roof of the building, according to a witness. More than a dozen bags of evidence were collected Saturday.