Image caption The portrait was probably taken a year or two before Billy the Kid was killed by Sheriff Patrick Floyd Garrett

The only known authenticated portrait of the notorious outlaw Billy the Kid has sold for $2.3m (£1.4m) at auction in Denver, in the US state of Colorado.

The tintype - an early form of photo using metal plates - is believed to have been taken in 1879 or 1880 in Fort Sumner, New Mexico.

It depicts the gunfighter in rumpled clothes and a hat, gazing at the camera and holding a Winchester rifle.

The tintype was bought by private collector William Koch.

It went for some six times the estimate - making it the most expensive piece ever sold at Brian Lebel's Annual Old West Show & Auction, said auction spokeswoman Melissa McCracken.

Lawless

Billy the Kid gave the image to a friend, Dan Dendrick, in whose family it has remained ever since.

It is a classic image of the American West, said Ms McCracken before Saturday's auction.

"There's only one photo of Billy the Kid, and I think that's why it captivates people's imagination," she said.

The outlaw was reputedly born in New York but moved to Colorado with his mother and brothers when his father died.

He fell into a career of thievery and lawlessness and was hunted across the southern US states and northern Mexico.

He is widely thought to have killed 21 people, although some sources put the figure as high as 27.

Billy the Kid was captured and sentenced to hang for the 1878 murder of a county sheriff. He then escaped, only to be hunted down and killed by Sheriff Patrick Floyd Garrett on 14 July 1881.