An explosion tore off both hands of a man who was mixing chemicals inside a Hyde Park apartment building this morning, Police Commissioner Edward F. Davis said.

The explosion occurred at about 9:07 a.m. in the basement of a four-story apartment building at 15 Riley Road.

Davis said the reason the chemicals detonated was so far unknown. Davis said the chemicals have been neutralized and it is now a “safe environment.”

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Police said preliminary investigation indicated the explosion was caused by a chemical experiment. Davis said the man might have been experimenting with fireworks.

Davis said the grievously injured man was taken to Brigham and Women’s Hospital for treatment.

Davis said there was no drug lab at the scene and there was no sign that terrorism was involved in the incident. He said police were checking whether the victim had a criminal background.

The commissioner said the victim, in his 30s, was working with chemicals, including a white powder, in the kitchen of his basement apartment when the blast occurred.

He said the victim had a roommate who was treated for respiratory problems after the blast.

A total of five people were treated after the blast, including three at the scene and two who were taken to the hospital, the police department said in a statement.

Resident Lindsey Bell said she was in her apartment in an adjacent building, when she heard an explosion and saw two men come out of the building, one of whom had his hands blown off and one of whom was in a towel.

She said she heard the man in the towel tell authorities a kitchen fire had caused the problem. At about the same time, “there was a lot of white smoke coming out of the building,” she said.

Evelyn Norman said was in her third-floor apartment when she heard a boom, a noise that she first dismissed as the sound of a passing truck.

“Then the moaning got my attention,” she said, “the moaning and the yelling.”

Norman said she left the building and saw a mostly naked man lying on a patch of grass in front. At least one of his hands was a bloody, mangled stump. “It was pretty bad,” she said.

As other residents helped the wounded man, a second man, also mostly naked, emerged. The man had a small patch of cloth draped around his waist, but was otherwise naked and appeared to be disoriented, she said.

The second man was not bleeding, she said. A neighbor provided a towel that Norman said she helped drape around the second man.

Later, Norman said, she saw a first responder was trying to get information from the first man, asking repeatedly what kind of chemicals he used. Norman said she did not hear the man answer, just moans of pain.

The second man was also being questioned, Norman said. He seemed reluctant to disclose what he knew. She said both were removed from the scene by emergency service personnel. A resident of the sprawling complex for 10 years, Norman said she had seen the two men over time but did not know their names.

Boston Fire Department spokesman Steve MacDonald said a small fire was ignited after the explosion, which was extinguished within a half-hour.

The police bomb squad spent more than an hour combing through the wreckage of the apartment.

The federal Bureau of Alcohol, Tobbacco, Firearms and Explosives and the FBI had responded to the scene by late morning. FBI spokesman Greg Comcowich said, “We’re aware of it and we are part of the investigation. Nothing has been ruled out.”

Neighbors said many police officers and firefighters reside in the building.

Police asked anyone with information to call CrimeStoppers at 1-800-494-TIPS (8477) or to text the word ‘TIP’ to CRIME (27463).