An Oklahoma Fourth of July parade turned tragic Thursday morning when a father accidentally ran over and killed his 8-year-old son, who had been on a parade float, police said.

Thousands of people had gathered on the streets of Edmond, Okla., to watch the 40th annual Fourth of July parade, which is part of the weeklong LibertyFest celebration.

Just before 11 a.m. (noon ET), the little boy "either jumped or fell off" of a flat-bed trailer that his father was driving for a local martial arts group, AKA Karate, according to Jenny Monroe, a spokeswoman for the Edmond Police.

The young boy was one of about 20 children on the float.

A nurse who was in a car behind the trailer tried to administer CPR, NBC station KFOR of Oklahoma City reported. When he was loaded into the ambulance, he still had a pulse, the nurse said, but he was pronounced dead when he arrived at UC Presbyterian Hospital.

Edmond police don't suspect foul play, calling it a "horrific, unfortunate accident."

Police haven't released the name of the boy or the driver, but they said they expect to have more information by Friday.

In Bangor, Maine, the driver of a tractor taking part in the city's Fourth of July parade was killed when an antique fire truck rammed it from behind, police said.

The parade was moving along an alternative route because police were in a standoff with a gunman along the original route, NBC station WLBZ of Bangor reported. It wasn't immediately clear whether the rerouting may have contributed to the accident.

The driver's identity hasn't been released.

A suspect was later taken into custody in the police standoff, WLBZ reported. The man, identified as Perrin Oliver, 43, was charged with felony criminal threatening and felony aggravated reckless conduct with a firearm.

Police said Oliver was firing from a second-floor apartment window when they arrived.