Image copyright AFP Image caption Somkid Pumpuang, pictured in an undated police handout, was spotted on a train

A couple who spotted a notorious Thai serial killer on a train because of the distinctive scar on his forehead have led police to arrest him.

Somkid Pumpuang, 55, was released early from prison in May, where he was serving life for murdering five people.

He was back on Thailand's wanted list on suspicion of killing a sixth person in Khon Kaen on Sunday.

Thai police launched a manhunt after the latest killing - that of a 51-year-old woman, Ratsami Mulichan.

How did they identify him?

The young couple, who have not been named for safety reasons, sat opposite Somkid after boarding the train at 08:37 (01:37 GMT) on Wednesday morning.

The woman noticed the passenger had a scar over his left eyebrow, which was the same as that in the photo used in wanted posters.

Image copyright EPA Image caption Police stormed the train at Pak Chong station and arrested Somkid

The couple left to sit elsewhere, but when the woman explained why she felt uneasy, her boyfriend went back to the seat to take a photo of the man - which he then sent to the police station.

As the train pulled into Pak Chong station at about 10:45, officers boarded the train and arrested Somkid.

The boyfriend said the scar "confirmed to me that the man was Somkid", he told Thai TV.

Who is Somkid Pumpuang?

Before his arrest, police described Somkid as a "dangerous man".

Officials allege that Somkid befriended Ms Ratsami on Facebook by pretending to be a lawyer, and moved into her home on 2 December.

Two weeks later, she was found dead in her home, the Bangkok Post newspaper reported.

Somkid was convicted for carrying out the original five murders in 2005. All of those victims were women who worked as singers in nightclubs, or as masseuses in hotels.

In 2012, an appeals court handed him a life sentence. However, he was released on parole in May this year for good behaviour while in prison.