TICONDEROGA -- Five people were injured Friday night when aerial firework shells exploded on the ground during the Ticonderoga Best Fourth in the North fireworks display.

A crowd estimated at more than 2,000 people was watching the fireworks show when three 6-inch aerial shells blew up inside launch tubes at 10:40 p.m.

Another firework shell then launched into onlookers and detonated in a burst of sparks.

"Witnesses saw an explosion by the show and seconds later an explosion into the crowd," Ticonderoga Town Police Investigator Daniel LaFrance said.

The show was about three-quarters of the way through and was being staged from the field next to Ticonderoga Little League Park, while a Fourth of July carnival was under way in nearby Bicentennial Park.

A fireworks technician employed by Pyro Engineering Inc., part of Bay Fireworks of Farmingdale, Long Island, was seriously injured by the explosions, LaFrance said.

John M. Michalak, 42, of Argyle had a severe laceration to his right leg, and multiple burns, cuts and scrapes. He was taken by Ticonderoga Emergency Squad to Moses-Ludington Hospital in Ticonderoga, then transferred to Fletcher Allen Health Care in Burlington.

Michalak was listed in serious condition Saturday at the hospital.

He apparently lit a fuse on an aerial shell that exploded in its plastic firing tube, setting off two other rounds in their tubes and knocking over another tube that fired into spectators.

"That (shell) went southwest of the show toward Burgoyne Road and injured three civilians," LaFrance said.

A 16-year-old boy from Ticonderoga and a 16-year-old boy from Port Henry were both treated and released at Moses-Ludington Hospital, while a 40-year-old male from Bennington, Vt., refused treatment.

A female employee of Pyro Engineering was also treated and released at the hospital for minor injuries, while a third employee, a male, declined treatment.

Their names were not released.

Plastic tubes called mortars were being used to direct the firework shells into the sky from the staging area.

"There's a wooden rack that contains three Kevlar tubes," LaFrance said. "One of the workers ignited the fuse and it blew up on the rack, causing all three to be destroyed. Fragments of wood and plastic and fireworks went over a 200-foot radius. It was a chain reaction, and it knocked down other tubing and set off a firework into the crowd."