BIDDEFORD — An overnight wave of suspicious fires here prompted Mayor James Grattelo on Thursday to ask residents for their help in solving the apparent arson case.

At least seven fires were set late Wednesday or early Thursday. No one was injured, but five families lost their homes in an apartment building fire.

Most of the fires were small — four were in garbage dumpsters while another burned in a curbside pile of trash.

But five families had to flee an Emery Street apartment building at 11 p.m. Wednesday when a fire set on the porch there raced up the outside of the building and engulfed porches and a third-floor apartment.

“It was just the screaming, `Fire! Fire! Fire!’ We didn’t smell any smoke at all, and then the whole thing went up. Boom!” recalled Ralph Mercier of Wednesday night’s fire at his girlfriend’s apartment.

Grattelo said he’d been in touch with fire and police officials, who have launched an aggressive investigation into the fires.

“We are still uncovering what took place during the night,” Grattelo said Thursday morning. “We’re taking this seriously, and we need the help of the community.”

“This is extraordinary,” Biddeford Fire Capt. Rene Lachance said of the fires.

But even as officials briefed the mayor, more residents were calling to report small fires on their properties that apparently burned themselves out during the night.

“Fortunately, most of them have been minor,” Grattelo said.

Assistant Fire Chief Robert Fournier said the dumpster fires were just the latest in a recent series.

“We’ve been having problems with dumpster fires since last Friday. We had two Friday, two Monday and then these last night.”