An unlicensed driver trying to pass a car on a snowy New Hampshire road died after crashing head-on into a car carrying four Secret Service agents on Democratic presidential candidate Hillary Clinton's protective detail, police said Wednesday.

The crash happened shortly after 7 p.m. Tuesday in Wakefield, near the Maine state line.

The agents were in a Ford Taurus heading south on Route 16, Wakefield police said. A northbound Mercury Sable with three people inside crossed a double yellow solid line and struck the agents' sedan, police said.

The Sable driver, Bruce Danforth, 45,of Ossipee, was killed. Police said an autopsy has been performed on him, but the results of blood analysis will take weeks.

Court records show Danforth had been charged seven times since 2010 with driving while his licence was revoked or suspended, including two occasions when he was also charged with drug-related offences.

He had pleaded guilty to — or been found guilty of — the charges, was fined hundreds of dollars and spent some time in county jail. After his most recent arrest, on Jan. 4 of this year, Danforth was ordered to pay $620. At the time of Tuesday's crash, the fine was unpaid and a hearing had been scheduled to consider a contempt charge.

Agents injured

A Secret Service spokeswoman said the agents, whose names weren't released, had serious but not life-threatening injuries.

Police Chief Kenneth Fifield said the crash caused "serious bodily injury" to all the passengers, including the two people in Danforth's car: Kristina Buswell, 21, and Natasha Meroski, 35.

State police Lt. Kevin Duffy said all four agents were wearing seat belts. "There was no improper driving at all on behalf of the Secret Service agents," he said.

He said police were trying to find the driver or a passenger in the car that Danforth was trying to pass to help with the accident reconstruction.

Clinton sends condolences

Clinton was campaigning at a town hall meeting Tuesday night in Berlin, about 125 kilometres north of Wakefield on Route 16.

Clinton said she was saddened and concerned to hear about the crash.

"My husband and I send our prayers and condolences to all the victims and their families," she said in a statement. "We are grateful every day for the service, dedication, and professionalism of the U.S. Secret Service."