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A spurned lover shot his ex-girlfriend, her boyfriend and two others dead at a US car wash, family members say, before turning the gun on himself.

Suspect Timothy Smith, 28, is on life support with a possible self-inflicted gunshot wound to the head, say police.

The four were shot dead on Sunday morning at Ed's Car Wash in rural Melcroft, Pennsylvania.

One of the victims, Chelsie Cline, 25, recently ended a relationship with the suspect, .

Ms Cline's half-brother, Seth William Cline, 21, Courtney Sue Snyder, 23, and William Scott Porterfield, 27, also died in the attack.

All four had just pulled up at the car wash in two vehicles.

Another woman survived the shooting with minor injuries from shattered glass, police said.

The gunman wore body armour and was armed with an AR-15 semi-automatic rifle, a 9mm handgun and a 308 rifle, according to Fayette County District Attorney Richard Bower.

Police have not divulged a motive for the killings.

Ms Cline's half sister that she believes she and the gunman used to be in a relationship.

But she said the suspect had become obsessed with Ms Cline, leaving beer on her car and sending her flowers.

She said the shooter seemed to be "kind-hearted", though he was obsessed with guns, according to the newspaper.

Police described Ms Cline as Mr Porterfield's "companion", .

Mr Porterfield's wife, Jenna Porterfield, 24, said police had told her he was cheating on her with Ms Cline.

Mrs Porterfield said she and her husband only married in November, but had been having difficulties.

"I'm not going to hold that against him," she told the newspaper of the alleged affair. "I'd give anything to have him back."

According to local media, four days before the shooting Ms Cline had shared a message on Facebook that said: "After this week, I (really) need to get taken out… on a date or by a sniper either one is fine (with) me at this point".

Mr Smith commented: "I could do both", according to local media.