Authorities are investigating an incident in which a sheriff's deputy says he witnessed a man with a gun threatening a young boy who was getting off a school bus in north Minneapolis.

A Hennepin County sheriff's deputy wrote in an affidavit for a search warrant that at the time of the incident on Monday he had been in an unmarked squad stopped at a red light at the intersection of N. 29th and Lyndale avenues in the Hawthorne neighborhood.

The boy, who the deputy estimated was 8 or 9 years old, hopped off the bus and was crossing the street in front of a black BMW 550i occupied by four men, the affidavit said.

The man in the front passenger seat rolled down the window and pointed a handgun at the youth, according to the affidavit. The young boy dived to the ground "for fear of getting shot" as the BMW sped away, the affidavit said.

The deputy wrote that he followed the BMW and radioed its license-plate numbers in to dispatchers, discovering that the car was stolen.

A Minneapolis police squad car soon joined the chase and tried to pull the BMW over, but "the vehicle sped away at a high rate of speed," the affidavit said. Authorities tried to keep up but eventually called off the pursuit for safety reasons, according to the affidavit.

The BMW was found abandoned at N. 24th and 4th avenues a short time later and was towed to the department's forensic garage to be searched, the affidavit said.

As of Thursday, detectives were still trying to identify the alleged gunman.