BOSTON — Workers have returned to a Boston federal courthouse that was evacuated because of a bomb threat amid reports of a breakthrough in the marathon bombings investigation.

Nikki Credic-Barrett, a spokeswoman for the U.S. Marshals Service in Washington, says a security sweep was conducted because of the bomb threat.

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A law enforcement officer wearing tactical gear stands guard outside the John Joseph Moakley Federal Courthouse, which was evacuated Wednesday in Boston. The Associated Press

Court workers got back inside the building about an hour after the evacuation.

Crowds of reporters are gathered outside. The FBI and the U.S. attorney’s office in Boston say no arrests have been made in the marathon bombing.

Attorney Francis DiMento says he was in a hearing when someone came over the loudspeaker and told everyone to get out. The courthouse has a day care attached and at least one crib was wheeled out.