A wind-fueled fire that ripped through a vacant Riverdale Heights house Friday night and injured seven firefighters — two of them critically — has been ruled arson, Prince George’s County Fire Chief Marc S. Bashoor said Wednesday.

“This cowardly crime is unconscionable to me,” Bashoor said at a news conference. “I assure you that we will leave no stone unturned to bring anyone respon-sible for this to justice.”

Several agencies, including the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, are investigating the blaze, along with an unreported fire at a neighboring house at least two weeks earlier.

Officials said the two fires appear related, but they released few details.

Friday’s blaze gutted a vacant house in the 6400 block of 57th Avenue, just east of Kenilworth Avenue. Flames had engulfed the basement when trucks from the Riverdale, Bladensburg and College Park fire departments arrived shortly after 9 p.m.

Seven firefighters entered through the front door, on the east side of the house. Strong winds out of the west were gusting up to 45 mph, and a blast of wind apparently entered the burning basement and sent a fireball up the stairs.

The firefighters on the first floor were engulfed by a blowtorch-like jet of flame, which shot out the front door, and all seven were burned. A group of firefighters in the basement escaped injury in what authorities described as a freak occurrence.

One of the injured firefighters, Ethan Sorrell, 21, suffered burns to his airways, and a breathing tube was inserted over the weekend. The tube has since been removed, and he was released from the hospital Tuesday, Bashoor said.

“Had the tube not been put in, he would have died,” he said.

A second firefighter, Kevin O’Toole, 22, remained in critical but stable condition in the burn unit at Washington Hospital Center, where he is being treated for second- and third-degree burns over 50 percent of his body. He faces weeks of recovery and months of rehabilitation, authorities said.

“Arson is not a victimless crime,” ATF Special Agent David Cheplak said.

Surrounded by fire and police officials and prosecutors from the Prince George’s state’s attorney’s office, Bashoor said he had ordered three investigations into the incident. One of them is a safety review to focus on the fireball that injured the seven firefighters and rattled the department.

“I can’t describe the significant emotional toll this has taken,” Bashoor said, standing near Sorrell’s scorched radio, which reeked of smoke, its knobs melted down to nubs. “I’ve seen grown men and women crying. It’s been very difficult.”

In investigating Friday’s blaze at 6404 57th Ave., officials discovered that the house next door — at 6402 57th, also vacant — had received fire damage weeks earlier. “We do believe the two incidents are related,” Prince George’s Fire Marshal Scott Hoglander said. He declined to elaborate further.

Despite the fires in adjacent houses and less than a month apart, the ATF’s Cheplak said he wasn’t ready to declare that a serial arsonist might be on the loose. But, he said, “if somebody does it once, there’s a chance it will be done again.”

Officials asked for help with the investigation, urging anyone with information about either incident to call Crime Solvers at 866-411-8477.

“Somebody out there saw something,” Hoglander said. “Your information is vital to our investigation.”