Three days after a local man and woman were shot in the Dorchester neighborhood of Boston, one victim was clinging to life and a team of investigators was searching for the shooter.

Boston police said Friday night Shameek Garcia, 32, remained in critical condition after suffering multiple gunshot wounds, at least one to the head.

Garcia was shot along with his girlfriend, 29-year-old Erica Field, as the pair sat in a car Tuesday night in a lot on Norwell Street. Field died of her wounds. Garcia has remained in critical condition since the victims were first discovered sitting side-by-side in their rental car.

By the end of the day Friday, no arrests had been made. Police from Maine and Massachusetts were joined by federal agents in their investigation into the shooting. In Maine, Lewiston police were assisting agents from the Central Maine Violent Crimes Task Force and their counterparts from Massachusetts. So were other departments in towns around Lewiston.

Part of the probe was expected to center on the gun used in the shooting, a weapon that has not yet been recovered.

Police said they have seen a longtime trend where criminals from lower New England states trade drugs for guns provided by criminals in Maine and New Hampshire. The reason is simple: strict gun laws in states like New York and Massachusetts make obtaining firearms difficult in those states. Meanwhile, guns are far more available in Maine through private sales, gun shows and theft.

“One of the most common means is the exchange of guns for drugs,” Maine Drug Enforcement Agency Supervisor Gerry Baril said. “There are more drugs in Massachusetts and more guns in Maine. It’s a supply-and-demand situation.”

The guns-for-drugs pipeline, Baril said, follows a similar path as the drug trafficking pipeline between Maine and population centers to the south, cities like Hartford, Conn., New York and the greater Boston area. How Garcia and Field fit into that network has not yet been established, police say.

Garcia is a known drug dealer from Lewiston, with state and federal convictions for gun- and drug-related crimes dating back more than a decade. He served prison time both federally and locally in the 1990s and more this decade. His most recent drug-trafficking conviction was in late 2006, when he was ordered to serve another two years in state prison. It was not clear how long Garcia had been free before he was shot Tuesday night.