By Joseph Menn

SAN FRANCISCO, June 30 (Reuters) - A Secret Service agent who stole money seized by the government in the investigation of underground drug bazaar Silk Road is now suspected of stealing money in at least two other cases, according to court filings unsealed on Thursday.

In the larger of those cases, he is thought to have been behind the theft of about $700,000 worth of Bitcoin from a Secret Service account three months after the agency was urged to block his access, the documents say.

Former agent Shaun Bridges pleaded guilty last year and was sentenced in December to nearly six years in prison for stealing more than $800,000 of the crypto currency Bitcoin during the Silk Road investigation.

According to an affidavit unsealed Thursday, the Justice Department learned in April 2015 that Bridges might have kept a private cryptographic key giving him access to a Bitcoin wallet with the $700,000 in currency that the Silk Road task force had seized in 2014. The department urged the agency to move the funds elsewhere.

"Unfortunately, the U.S. Secret Service did not do so and the funds were thereafter stolen, something the U.S, Secret Service only discovered once it was ordered by a court to pay a portion of the seizure back to affected claimants," a team of prosecutors wrote in an accompanying motion. The Bitcoin in question was moved in July 2015 but only discovered missing in December, the affidavit said.

The Secret Service and Bridges' attorney Steven Levin declined to comment.

In the previous case, Bridges admitted he stole money from Silk Road accounts and framed someone else for it, leading Silk Road chief Ross Ulbricht to plan a murder. Ulbricht is now serving a life sentence.

(Reporting by Joseph Menn; Editing by Cynthia Osterman)