A vocational college student has been arrested for murdering his ex-girlfriend in a jealous rage and attempting to murder her 11-year-old sister then leaving her for dead.

Dusit Panitchayakarn College first-year student Atthit Phromchan, 20, convinced his former girlfriend Supawadee Waengwan, 19, to meet him at his Lat Phrao home on Sunday, where he allegedly shot her.

Supawadee's 11-year-old sister, who cannot be named, heard the gunshot and rushed into the home, where Atthit knocked her unconscious and attempted to strangle her with wire.

Atthit, his sister Jenjira, 18, and friend Maew Jaksupong, 26, carried the victims to an estate refuse dump and left them there.

But Supawadee's young sister was not dead; she regained consciousness, then she sought help from a resident and alerted police.

Police arrested Atthit and Maew yesterday at the New Mor Chit Bus Terminal on their return from Chon Buri.

Atthit has been charged with murder and illegal possession of a weapon. Atthit, Maew and Jenjira have been charged with concealing a body.

The Ratchadaphisek Criminal Court remanded Atthit in custody for 12 days pending further investigation and post-mortem results. Police opposed bail because they fear Atthit is a flight risk.

Atthit later told reporters he had tried to reconcile with Supawadee who had left him three months ago for another man. She refused and in a jealous rage he dragged her into the bathroom where he shot her. He said he attempted to kill the sister because she had witnessed the crime.

He called his friend and sister for help to dispose of his victims and then he and Maew fled to Chon Buri. They decided to return to Bangkok yesterday.

"I deeply regret this but I got carried away. I'm an intensely emotional person. I never meant to hurt her sister but she saw what happened and I couldn't let her go," he said.

He dismissed the sister's statement to police he killed Supawadee, a first-year Suan Sunantha Rajabhat University student, because he owed her money.

"We're both working and studying and we used money from the same pocket. I killed Supawadee because I couldn't deal with her cheating on me and leaving me for a new guy. I apologise to her family and young sister," he said.

The sister told police Atthit killed Supawadee after the two argued over a debt. Shortly after Atthit attacked her she heard people discussing how they would dispose of the bodies. She later regained full consciousness at a Senaniwet 2 Housing Estate dump.

Police found blood in the bathroom of Atthit's Soi Phaholyothin 30 home and a shotgun and cartridges under a mattress. Jenjira led them to Supawadee's body.

Police arrested Atthit after he called his mother from a public phone booth at the bus terminal.

The killing is the second by a jealous student this month. Last week a Nakprasit Wittayalai School student shot his ex-girlfriend dead in a Nakhon Pathom classroom and then tried unsuccessfully to take his own life.