A boat exploded on a canal near the Maryland and Delaware border on Saturday, injuring at least nine people, officials said.

The explosion happened on the Chesapeake and Delaware Canal around 4 p.m., right after the 32-foot-long boat had fueled up at Schaefer’s Canal House, a restaurant and marina in Chesapeake City, Md., said Vincent Hunter Dortenzo, a spokesman for the Maryland Natural Resources Police.

Four people were airlifted by the Maryland State Police with serious burns and five others were taken in ambulances to medical centers, Mr. Dortenzo said.

Seven of the injured people had been on the boat at the time of the explosion, according to the Office of the Maryland State Fire Marshal. The other two had been on a nearby dock.

The explosion drew a response that included helicopters, paramedics and more than a half dozen fire companies.

The cause of the explosion is under investigation, Mr. Dortenzo said, adding that the boat that exploded was a Wellcraft model from 1988. It was tied up to a dock at the time of the explosion, according to Volunteer Fire Company No. 1 of Chesapeake City.

It was one of at least three boating accidents in the United States around the Fourth of July holiday weekend, a popular time for boating.

In Maryland, two adults and two children were hospitalized on Friday after a boat caught fire on the Patapsco River, The Baltimore Banner reported.

And in Wisconsin, three children died on Friday after a boat capsized on Geneva Lake during a severe storm that brought powerful winds and waves.