Police who blocked traffic for protesters who shut down Interstate 35W in Minneapolis last week said they didn’t have advance warning the group planned to march down one of the state’s busiest freeways in the middle of the afternoon.

About 150 demonstrators took to the freeway on Dec. 4, protesting a grand jury’s decision not to indict a police officer in the death of a black man. The group marched for 3 miles from south Minneapolis to downtown, where they entered City Hall and climbed the stairs to the offices of the City Council. The freeway was shut down for more than an hour.

None of the protesters was arrested or cited. Rather than make arrests, Minneapolis Police Department and State Patrol officials said they made on-the-spot decisions to keep demonstrators safe from traffic. The city does not issue permits for demonstrations, but Minneapolis police do have a written policy for dealing with such events: keep the peace and don’t interfere unless a crime has been committed.

It was that policy that led some organizers of the Dec. 4 demonstration to believe they’d be able to take to a busy freeway without prompting a confrontation with police.

Michael McDowell, an organizer with the group Black Lives Matter Minneapolis, said activists began making plans for the event Dec. 3. Many had been involved in other demonstrations following a grand jury’s decision not to indict the police officer involved in the shooting death of Michael Brown in Ferguson, Mo. After a New York grand jury declined to indict an officer in the death of another black man, Eric Garner, the Minneapolis demonstrators wanted to make an even bigger statement.

A few protesters volunteered to drive onto the northbound lanes of I-35W and work together to slow traffic, eventually stopping it all together. At the same time, leaders of the demonstration that began at 34th Street and Nicollet Avenue announced to the crowd that they planned to head toward a nearby on-ramp. Police said they learned of the plans at the same time.

As the group moved onto the freeway, officers told leaders to stop but didn’t intervene when the crowd ignored the orders.

By the time the group was on the freeway, state troopers were moving in to stop traffic. They also announced, repeatedly, that protesters should leave the freeway at the first exit. Only a few did. McDowell said police hadn’t hassled protesters who shut down Hwy. 55 in an earlier protest, so he didn’t expect a confrontation on the freeway.

“They were saying: ‘Exit at Lake Street,’ but that wasn’t the plan,” McDowell said. “We wanted to hold traffic for a while.”

Protest organizer Mica Grimm said she didn’t expect the troopers would continue to follow the protesters through the march, or that they’d help by blocking on-ramps. At various times during the march, she believed troopers would start arresting demonstrators.

Capt. Eric Roeske, a spokesman for the Minnesota State Patrol, said the decision to continue escorting protesters to downtown was made within his agency and didn’t require outside approval.

Still, he said, “this is not something we endorse or support.”

Grimm and McDowell said the group plans more demonstrations.