A man claiming to have a bomb has taken two people hostage at a Hillary Clinton campaign office in Rochester, New Hampshire, police say.
Mrs Clinton is at a Democratic party event in Virginia and is not involved in the siege.
Unconfirmed reports later said all the hostages had been freed.
The area has been evacuated and police have sent a tactical bomb unit to the scene, according to a New Hampshire police spokesman.
Law enforcement sources have told the BBC that the suspect is well-known locally, and has a history of emotional issues.
He had earlier told his son to watch the news, according to sources.
Hostage negotiators have arrived at the scene, police said.
The man released a mother and her baby but held onto two volunteers, according to police.
Local shopkeeper Lettie Tzizik told local television station WMUR TV: "A young woman... came rushing into the store just in tears, and she said: 'You need to call 911 - a man has just walked into the Clinton office, opened his coat and showed us a bomb strapped to his chest with duct tape'."
The offices of rival presidential candidates Barack Obama and John Edwards have also been evacuated.
Mr Obama's office is on the same street as the Clinton campaign headquarters.
New Hampshire is due to hold the first primary of the 2008 presidential election campaign in January, and all of the candidates are currently concentrating much of their campaigning efforts in the state.