PLYMOUTH — When a barge carrying the town’s July 4th fireworks burst into flames mid-show, consuming many of the projectiles, disappointed viewers figured that was that.

But new dangers appeared Tuesday morning as unexploded shells washed up on popular beaches.

The town warned beachgoers to watch out for the shells, which could explode and seriously injure them. By afternoon, crews in Plymouth recovered a “pickup truck’s-worth’’ of unexploded fireworks along the coast, said Ed Bradley, the fire chief.

Thousands of spectators had flocked the Plymouth shores Monday, but rain and foggy weather and a return to the workweek Tuesday morning thinned out the crowd. As the clouds lifted and people filled the shorelines, many beachgoers shrugged off the news that dangerous fireworks could wash up next to them.

“It was an accident, no one died and that’s all that matters,’’ said Lee Truschelli, 36, who spent the afternoon fishing off a shoreline with her three young boys. “It’s pyrotechnics. Things like this happen.’’

“I just told my girls that if they see anything they don’t recognize, just don’t touch it,’’ said another unfazed resident.

The fire broke out around 9:30 p.m. during the town’s annual Fourth of July Celebration. A second barge nearby caught fire about 35 minutes later.

The fires caused hundreds of fireworks to fall into the water. Even when soaked, the shells could be dangerous.

“Professional fireworks are wrapped in such a way that they are somewhat waterproof so that they can be used in the rain,’’ Bradley said. “Once they dry out they can become a little unstable.’’

By day’s end, crews had collected nearly the entire supply that had fallen overboard. Most were still intact, officials said.

Bradley said there were about $35,000 worth of fireworks for the show, but only about 20 percent burst into the sky before the fire. The rest either burned or fell into the water.

A preliminary investigation suggested that the first barge fire started when a firework shell exploded inside a mortar tube, Bradley said. The second barge fire likely started when a battery involved in detonating the fireworks became overheated.

There were no injuries, as the operator was controlling the display from a service pontoon boat more than 100 feet away from the barge. The Massachusetts fire code requires that fireworks displays be ignited electronically instead of manually, the state fire marshal’s office said.

The office was also investigating another fireworks mishap, this one in Salisbury, where a pile of boxes used to carry fireworks ignited on a barge. A town fire boat quickly put out the flames, which did not ignite any fireworks.

“These incidents underscore just how unpredictable fireworks are, even for trained professionals,’’ said State Fire Marshal Peter Ostroskey.

In Plymouth, Bradley said the town’s fireworks contractor, Atlas Fireworks from New Hampshire, has put on the Plymouth show for some 30 years without incident. Atlas officials could not be reached for comment.

“Everything was done in the correct way — the racks were secured as they were supposed to be, they were the right distance apart,’’ Bradley said.

While many beachgoers were unbothered by the prospect of fireworks washing on shore, others were frustrated that their July 4 tradition had been cut short.

Jacqueline Tedesco, 21, was watching the fireworks with her parents when she caught the explosion on video. At first, she didn’t know whether it was intentional — it was hard to tell when the fireworks ended and the fire began.

Her mother, Colleen, described the scene in a few words:

“Fireworks, and explosion, then silence,’’ she said.

Heather Stockbridge and her family have come to Plymouth for the fireworks for the past decade. On Monday, staying at a hotel right on the harbor, they had a perfect view of the show — and the fire that soon followed.

“They started off normal, and then they were shooting off in all different directions,’’ she said. “We thought it was the grand finale.’’

Andy Rosen and John R. Ellement of the Globe staff contributed to this report. Trisha Thadani can be reached at trisha.thadani@globe.com. Follow her on Twitter @TrishaThadani