Authorities partly closed a downtown Minneapolis freeway briefly Tuesday morning as they made a “high-risk” traffic stop to detain a motorist who allegedly brandished a gun at another driver.

Turns out the driver was an agent for the Bureau of Alcohol, Tobacco and Firearms, a State Patrol report said.

The incident began about 6:40 a.m. on eastbound I-694 in Brooklyn Park, when a motorist called police to report that another driver was displaying a gun.

Police caught up to the driver on I-94 at Olson Highway and blocked two southbound lanes long enough that traffic backed up to W. Broadway. The State Patrol took the man into custody but released him after officers learned he was an ATF agent who was carrying proper credentials.

“It’s under investigation,” said Lt. Eric Roeske of the Minnesota State Patrol. “We’re trying to determine the facts of what occurred.”

The agent’s name was not released per Minnesota Statute 13.82, which allows the identity of an undercover law enforcement officer to be kept private.

An ATF spokesman said Tuesday afternoon that the incident was under review.

TIM HARLOW and JOY POWELL