PORTLAND – More than a dozen firefighters and paramedics responded to a mock biochemical attack at the Presumpscot School, an exercise designed to train emergency crews to deal with terrorism.

The 21/2-hour exercise began Thursday when someone opened an envelope addressed to a school club, releasing a cloud of white powder. That set off fears that the powder was toxic and prompted a call to 911.

Firefighters arrived at the scene and began to decontaminate those exposed to the powder. Eventually, there were five firetrucks and one ambulance parked outside.

“With the events of September 11, we’re certainly changing our focus,” said Acting Fire Chief Fred LaMontagne. “This is a small portion of our training to be prepared for a domestic terrorist attack.”

The drill was the first of its kind to be scheduled in Maine after Sept. 11, said Art Cleaves, head of the Maine Emergency Management Agency, who commended Portland officials for their initiative. “Many of the towns are doing forums where they talk about preparedness, but they haven’t stepped forward to do the full-blown exercise,” Cleaves said.

Volunteer actors played the role of victims, who were told to put on lightweight plastic suits over their clothes as well as face masks and rubber gloves. Six of the firefighters donned similar gear and carried air tanks on their backs.

A stun grenade thrown into a trash can caused a sudden explosion outside the school, reminding firefighters that secondary terrorist devices could be present.