The Bronx house explosion that killed a fire chief this week was caused by a gas main that had been tampered with. NYPD Commissioner James O'Neill told reporters that a spark ignited the leaking gas and caused the explosion that killed Battalion Chief Michael Fahy on Tuesday.

"So far as for the crime scene, it is active as of right now. We have recovered an enormous amount of material, pipes and meters that will be taken to Maryland under the ATF and examined for tampering," NYPD Chief of Department Robert Boyce said at the press conference.

Police have arrested Garibaldi Castillo, 32, and Julio Salcedo Contrer, 34, in connection with the blast. Castillo is facing marijuana possession charges, and the authorities are looking into whether the house was being used to grow weed commercially. Prosecutors are also weighing whether to charge Castillo with murder. Contrer, who was arrested in Cliffside Park, New Jersey, was wanted for questioning, and has waived extradition to New York, speeding up the process of moving him here.

Castillo is being held without bail. Bronx prosecutors accuse him of being at the Kingsbridge home the day of the explosion, and dumping a cellphone linked to the address immediately afterwards.

Castillo's lawyer, Francisco Serrano, argued at a bail hearing that the state has not established anything concrete tying Castillo to the house. He also said the pot plants were too young to be smokeable.

"They haven't said that he owns that place, that he rented that space, that he put water into these plants, that the plants are mature," Serrano said.

Prosecutors say Castillo was arrested with the house's front door key in his pocket.

Inside the house, police found marijuana seedlings in the basement, and larger plants on the ground and second floor of the two-story brick house, along with heaters, liquid fertilizer, and tanks of helium, according to an assistant district attorney who spoke in court. The fertilizer is highly flammable. It's not clear if the alleged gas line tampering was connected with the alleged weed-growing operation, but it could have been. As NY1 reports:

Law enforcement sources tell NY1 that tampering with the gas line would enable a grower to avoid suspicion that a lot of gas is being used and avoid paying a high gas bill.

Firefighters responded to the house around 6:30 a.m. after a 911 call came in about a gas leak at the address, 300 West 234th Street. Smelling gas, and seeing the plants, they called police and Con Edison. Salcedo tried to flee on foot, and in his car, according to prosecutors.

The explosion occurred about 40 minutes later, as Fahy was leaving the house with officers. He was struck in the head by falling debris and died, according to the fire department.

"The explosion had so much force that it blew the roof completely over the power lines and over the tops of trees to land in the middle of the street," prosecutor Christine Scaccia told the judge.

Mourners gathered for Fahy's funeral on Thursday in Yonkers. The FDNY has posthumously promoted him to the rank of deputy chief, the highest civil service promotion in the department.