University of Michigan law student Sam Rubinstein was attending a campus vigil for victims of the New Zealand mosque shooting Saturday when he heard someone yelling, “MOVE! MOVE!”

He turned to see a police officer running toward the group. Panic set in as the crowd started moving away and up the stairs of the UM Hatcher Graduate Library. He and others he was with ended up huddling in a third-floor bathroom for an hour.

“There could have been no real threat at all or it could have been a very real threat,” said Rubinstein, 24, of New Jersey.

It turned out to be no threat. But for thousands across Michigan, a 4:40 p.m. alert from the University of Michigan, apparently sparked by popping balloons, set off three hours of fear and anguish. Those who scrambled from the perceived threat barricaded themselves in classrooms and bathrooms and were finally assured at 7:50 p.m. that the threat was false.

Campus counselors are being made available Sunday because of potential trauma suffered during the episode.

"UM EAlert Ann Arbor: Active shooter in Mason Hall. Run, hide, fight," was the full content of the initial alert sent by the University of Michigan division of public safety at 4:40 p.m.

That alert was followed 37 minutes later by an alert that, for the first time, said the report was unconfirmed. Many had already scrambled into hidden corners for safety.

University police said they received as many as 20 reports at 4:35 p.m. of shots fired in Mason Hall. Officers told The Detroit News a short time later that they have found no evidence of shots fired or an active shooter. Mason Hall is an academic building on The Diag.

Students were advised to run, hide and fight if necessary. Many barricaded themselves in bathrooms and rooms in the Hatcher library.

At 7:50 p.m., the school gave the all-clear saying the active-shooter alert was a false alarm.

Officers had secured the building and the situation remained under investigation by campus police and Washtenaw County Sheriff's Office, the school said.

In the meantime, students were using social media to communicate with the university and family for information.

U.S. Rep. Debbie Dingell, D-Dearborn, was among those at the Saturday afternoon vigil and her statement reflected the uncertainty of those moments.

"I attended the vigil," said her post, as reported by pressandguide.com, "and there now is an active shooter on campus.

"People gathering for peace, to fight this hate and madness....now they have been knocked down by panic and are hiding in bathrooms, and classrooms.

"We don't have facts ... We will see what the next hours bring.....this reality has me shaken to the core...... D2"

Rubinstein and others ended up in an adjacent building, the Shapiro Undergraduate Library. The group that he was with decided to barricade themselves so they took chairs and went into the women’s restroom on the third floor, where one student tied the hinge of the door shut. They went into stalls and stayed quiet for nearly an hour.

“It was a very strange feeling,” Rubinstein said. “I felt safe; at the same time I was scared.”

In a press release later Saturday, the University of Michigan said the calls to police about anactive shooting were prompted by "balloon popping activity" in the area that had no malicious intent.

After being cleared by police, Rubinstein left and planned to go back to his off-campus housing with more of a personal understanding of an active shooter situation.

“We shouldn’t live like this,” said Rubenstein. “At the same time I was experiencing it, there was a weird normalcy to it. You say shooter on campus and that triggers a certain response in people’s brains. People understand what it is. In some ways that was helpful because people were really prepared to deal with it. On the other hand, it shouldn’t be so normal that we need to be thinking in those ways.”

Staff Writer Kim Kozlowski contributed to this report.

srahal@detroitnews.com

Twitter:@SarahRahal_