Twelve people have said they witnessed a local police department's fatal shooting of a black motorist over the weekend, but none has stepped forward to say what they saw, a Florida sheriff investigating the death said Monday.

St. Lucie County Sheriff Ken Mascara said he and his detectives passed out cards, but no one has called.

"We need our community's help," Mascara said. "We aren't interested in rumors or gossip; we are looking to find the truth."

Fort Pierce Police Chief Diane Hobley-Burney asked the sheriff's office to conduct the investigation into Saturday night's shooting of 21-year-old Demarcus Semer so that it would be impartial, Mascara said.

The sheriff said Semer had just begun work at a tech company after working as a bank teller, had no criminal record and was "well-respected in the community." Semer also played quarterback and receiver at Fort Pierce Central High in 2012 and 2013.

Mascara did not identify or reveal the races of the two officers, but described them as well-respected veterans. He said they would be interviewed by Wednesday. He said the officers were not wearing body cameras and their cars were not equipped with dashboard cameras.

The confrontation began after an officer tried to pull Semer over for a traffic violation and he wouldn't stop until a second officer arrived, Mascara said. While stopped and talking to the officers, Semer began pulling away, clipping one officer in the leg and dragging the other in the car window, he said.

Semer eventually pulled over again, got out of the car and tried to flee, at which point both officers opened fire, Mascara said. Semer died at the scene. The sheriff said he didn't know why Semer would have tried to flee. He said no weapons or drugs were found on him, but investigators are getting a search warrant to inspect the car, hoping that will provide some explanation for Semer's behavior. Autopsy results are also pending, he said.

Mascara said his detectives will conduct a fair, open and thorough investigation. He said the results will be turned over to local prosecutors, who will decide whether to bring charges against the officers to a grand jury.

Josh Shaffer, the Fort Pierce Central football coach, was quoted by Treasure Coast Newspapers as saying Semer was "a brilliant young man who was so personable."

"He was a real leader who could make a connection with about anyone," Shaffer said. "He cares about everyone."

Semer's grandfather, Elijah Smith, of Fort Pierce, was quoted by the newspaper as saying that Semer had been with a group of friends practicing rap music before the shooting.

"He was a nice, good young man," Smith said.