By Evan Perez and Holly Yan
(CNN) -- The suspect at the center of a days-long manhunt was arrested near the Golden Gate Bridge, accused of having explosive materials in his home, an FBI source said Monday night.
Ryan Kelly Chamberlain II was arrested in Crissy Field, a scenic waterfront area where the iconic bridge spans the Pacific entrance to San Francisco Bay, a U.S. law enforcement official said.
Authorities had been looking for Chamberlain, 42, after searching his neighborhood Saturday.
"We don't know the intent or motive yet," FBI agent Peter Lee said over the weekend, stressing there was no threat to public safety.
But social media postings might provide some clues.
According to CNN affiliate KPIX, Chamberlain posted an apparent suicide note to friends on Facebook titled "Goodbye." The letter describes a history of depression, despair over a broken relationship and career problems.
"In the span of a few months, everything that mattered me to me betrayed me," the writer said.
But the FBI is not calling the letter a "suicide note," a spokesman told CNN.
Peter Lee, the FBI's spokesman in San Francisco, said investigators are trying to verify the origin of the post.
Lee added that there was no ricin found in Ryan's home, contrary to some reports.
His block was closed for some 15 hours over the weekend as investigators searched and cleared the area, CNN affiliate KTVU reported.
Howard Epstein, a former chair of the San Francisco Republican Party, told KTVU that Chamberlain worked with the local GOP before switching to work on the Democratic side.
"The only one I know that he worked with after he left the party was Gavin Newsom," Epstein told KTVU. "He was an organizer when Gavin ran for mayor."
In the letter, the writer says he's had some positive moments in the past few months.
"But so much was broken from this past year-and-a-half, and from moments way back before that, I guess it was insurmountable, and the time's up," the writer says. "Thank you. I'm sorry. I love you."
CNN's Artemis Moshtagian and Dana Ford contributed to this report.
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