ANN ARBOR, Mich. (AP) — A University of Michigan sorority whose members popped balloons as part of a team-building event has apologized for triggering “active shooter” alert on campus.

The balloon popping occurred March 16, at the same time a vigil was being held nearby for victims of the New Zealand mosque attacks a day earlier.

The alpha Kappa Delta Phi chapter issued an apology in a letter published Sunday in The Michigan Daily. Member said they’re “truly sorry” and should’ve been more “more cognizant” of their actions.

Alerts issued by the Ann Arbor school told students near Mason Hall to “run, hide, fight.” Authorities later said there had been “balloon popping activity” in the area and there was no “malicious intent” behind the activity.

The letter says sorority members stand “in solidarity with our fellow Muslim students and marginalized communities.”