A group of gang members who trafficked drugs and Oliver James Montgomeryguns and used dating websites to connect with people interested in hiring prostitutes were responsible for a series of robberies that led to four deaths, federal prosecutors said Friday.

Dubbed the “fentanyl robbery gang,” the group worked from New Hampshire to Virginia, according to Gerard Karam, the U.S. attorney for the Middle District of Pennsylvania. He said the gang members and their associates would arrange to visit a victim’s location with the intent to rob them of guns, cash, cellphones, identification documents, debit and credit cards, drugs and other items of value.

The victim would be offered drugs, usually purported cocaine, but were not told the narcotics contained fentanyl. If the victim refused the drugs, the gang members and/or their associates would forcibly administer them or surreptitiously introduce them it into their bodies, Karam said. This was done to incapacitate the victims so it was easier to steal from them.

If a victim wasn’t incapacitated, or not incapacitated quickly enough, the group would commit home-invasion robberies where they would be let into the victim’s home by other gang members or associates and then steal items of value at gunpoint or through other violent means, including threats and beatings.

The group is linked to three deaths in Pennsylvania — in Berks and Luzerne counties — and one in Portsmouth, New Hampshire, and prosecutors said it is likely there are other victims.

Seven group members have been indicted on numerous charges including kidnapping, aggravated identity theft, conspiracy to distribute drugs, distributing fentanyl resulting in death and serious bodily injury and weapons charges. Some group members were affiliated with New York City gangs, Karam said.

The investigation involved several state, county and local law enforcement agencies in Pennsylvania and New Hampshire, as well as the FBI and the U.S. Marshal’s Service in Scranton, Pennsylvania.