The Fairfax County Fire Department got the call at 2:39 p.m.: an explosion at Tysons Corner Mall. A firefighter had been injured.

Reporters, firefighters and police rushed, with their various expectations, to the scene outside Bloomingdale's Delicacies.

Yellow tape marked the area where a huge yellow fire engine sat. Investigators combed the scene. Shoppers stopped to inquire.

The situation appeared grave.

An 18-year-old man had been taken away for questioning after the explosion, which sent firefighter Todd Heilman to Fairfax Hospital with cuts to his face and glass in his scalp.

Was there a bomb? Several questions needed answers.

But within hours, as details emerged, the grave situation became much less grave.

It all started when firefighters responded to an alarm at the mall.

While they were investigating, a teenager approached one of them, making conversation and asking what was going on.

Then, the backpack slung over the young man's shoulder exploded.

"It was a very loud explosion," said Pam Weiger, a fire department spokeswoman.

What was thought to be a bomb turned out to be a soda pop bottle. The fuse turned out to be dry ice.

Jane Welch, a spokeswoman for Fairfax Hospital, said Heilman was treated and released. She said medical personnel referred to the firefighter's injuries as a "boo-boo."

The 18-year-old, whom police identified only as a man who worked in a photo store in the mall, was still being held for questioning last night.

A mall employee said the 18-year-old was in the back room experimenting with dry ice, which is delivered to the store to keep special film cold.

"He was freaked out about it," she said. "There was smoke everywhere.

"He just thought it was the coolest thing in the world and he wanted to take it home," the employee said.

Staff writer Thomas Heath contributed to this report.