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

The accident 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 over a double yellow solid line and hit the agents' Taurus.

The Sable driver, Bruce Danforth, 45, died. Police said an autopsy has been done on him, and they were awaiting blood-analysis results, which would take weeks.

A Secret Service spokeswoman said the agents sustained serious, but non-life-threatening injuries. Their names have not been released.

Police Chief Kenneth Fifield said the accident caused "serious bodily injury" to all the passengers, including the two people in Danforth's car, Kristina Buswell, 21, and Natasha Meroski, 35. Their conditions were not immediately known Wednesday afternoon.

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 was campaigning at a town hall meeting Tuesday night in Berlin, about 80 miles 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."