PROVIDENCE — The police have found the man who, they say, installed or helped to install the boiler for a residential heating system that is implicated in the deaths of three people from carbon-monoxide poisoning.

He is Alex Menendez-Mendez, 23, an illegal immigrant from Guatemala, Maj. Stephen Campbell, commander of the Police Department Investigative Division, said yesterday.

In an interview with detectives, Menendez-Mendez “admitted that he … did in some way participate in the installation,” Campbell said. But Campbell would not divulge what Menendez-Mendez said he did in the installation and whether he did it alone.

Menendez-Mendez is a witness in the police investigation of the deaths, which continues in close cooperation with the attorney general’s office, Campbell said. The police won’t say if they are conducting a criminal investigation, but in certain circumstances, someone whose negligence causes a death may be charged with a crime.

After the interview Thursday, Menendez-Mendez was turned over to agents for the U.S. Bureau of Immigration and Customs Enforcement, who now have him in custody. Menendez-Mendez entered the United States illegally about three years ago, according to Campbell, who would not discuss the circumstances of the man’s entry.

Sonia Flores, 46, a factory worker and longtime city resident who emigrated from El Salvador; her son, Ryan, 14, a ninth-grade student at the Met School branch downtown; and her live-in boyfriend, Marco Herrera, 52, a porter at a Cranston auto dealership and a naturalized U.S. citizen from Guatemala, died in Flores’ house at 345 Blackstone St., South Providence.

State and city authorities say the natural gas-fired boiler was incorrectly vented when it was installed on New Year’s Eve, allowing carbon monoxide fumes to back up into the house to a lethal level. The victims died that night or the next night, according to the police, but they were not discovered until Monday.

After hearing that the police were looking for him, Menendez-Mendez came to police headquarters at noon Thursday, according to Campbell. The major would not disclose details of the interview, how Menendez-Mendez came to be involved in the boiler work, or the man’s occupation.

He did say, however, that Menendez-Mendez and Herrera were acquainted prior to the work being done.

As part of their investigation, the police removed from the house the boiler and attached expansion tank, vent piping, and hot-water tank. Campbell said the hot-water tank is a less important part of the investigation than the boiler.

The replacement boiler that was installed on New Year’s Eve is a used boiler, but the police won’t say where it came from.

gsmith@projo.com