A Vietnam War veteran allegedly gunned down an illegal immigrant — accused of being part of an international construction conman gang — whom he had hired to work on his Philadelphia home, according to officials and reports.

George Barr, 75, allegedly shot and killed 20-year-old Salis Hanrahan, of Essex, England, around 2 p.m. last Wednesday following an argument inside Barr’s home in the Roxborough neighborhood, police said.

Hanrahan was part of a crew installing siding on the home, neighbors told NBC-10 Philadelphia, adding that they heard shouting before the young worker stumbled out of the house and collapsed from a gunshot wound to the chest.

Investigators have not revealed a motive in the killing — but federal officials, on Tuesday, released new information about the UK migrant that could provide some clues.

Hanrahan was allegedly a part of the “Traveling Conman Fraud Group” — a group of British and Irish scammers in the US illegally who overcharge clients after they fail to complete shoddy construction work on various buildings, the Department of Homeland Security told the outlet. 

DHS also confirmed the slain contractor was in the US illegally after being denied a visa for his affiliation with the traveling conman group, which has been deemed a transnational criminal organization by the FBI.

The extent of Hanrahan’s involvement in the gang is not immediately clear.

The alleged conman was shot in the chest, but managed to run out of the house before collapsing on the ground and succumbing to his injuries in the hospital, according to cops and neighbors.

His accused killer was described by a shocked neighbor as a polite and kind person who ran the unofficial neighborhood watch.

“He’s always friendly. Every time I’ve encountered him,” neighbor Erin Kelly told NBC-10 Philadelphia. 

“It’s just crazy that it happened here. It’s always quiet here, so for something to happen like this, it just shocked us all.”

Construction workers had been working on the house for days without issues in the quiet Roxborough section of Philadelphia, neighbors said.

Barr, a retired firefighter, was arrested and charged with murder, possession of an instrument of crime and recklessly endangering another person two days after the fatal shooting, according to court documents obtained by The Post. It is not clear if he has entered a plea.

The incident remains under investigation.

The Philadelphia Police Department declined to offer comment and the UK Home Office was not immediately available.