Image copyright Getty Images Image caption Walter Palmer's dental practice in Minnesota has been besieged by angry protesters

US officials have launched an investigation into the killing of a lion in Zimbabwe but say they have been unable to reach the American involved.

The US Fish and Wildlife Service (FWS) said it was "deeply concerned" about the "tragic" death of Cecil the lion.

Director Dan Ashe said they will "go where facts lead" but efforts to reach Walter Palmer have been unsuccessful.

Mr Palmer says he thought the hunt was legal but two Zimbabwean men have been arrested over the popular lion's death.

The dental practice he runs in Minneapolis has been closed since he was named as the tourist who shot Cecil, Zimbabwe's most famous lion.

Protesters gathered outside the building on Wednesday, carrying placards saying "Justice for Cecil", "Trophy hunters are cowards" and "Prosecute poachers".

On Thursday, the White House said it would review a public petition to extradite the American dentist after more than 100,000 signed it.

But spokesman Josh Earnest said it was up to the US justice department to respond to any extradition order.

What Cecil the lion means to Zimbabwe

When is hunting not poaching?

Image copyright EPA/Facebook Image caption Walter Palmer has faced a huge backlash online over his killing of Cecil the lion

Image copyright Reuters Image caption A petition calling for the American dentist to be extradited has more than 100,000 signatures

Earlier, the FWS said: "We are currently gathering facts about the issue and will assist Zimbabwe officials in whatever manner requested."

"At this point in time, however, multiple efforts to contact Dr Walter Palmer have been unsuccessful," it said, saying Mr Palmer or his representative should contact them immediately.

"It is up to all of us - not just the people of Africa - to ensure that healthy, wild populations of animals continue to roam the savannah for generations to come," the statement said.

The whereabouts of Mr Palmer is currently unknown, but he is thought to have returned to the US after Cecil was killed on 1 July.

'Life-long hunter'

In a letter to his patients, the dentist said he would assist authorities in Zimbabwe or the US in their inquiries and apologised for the disruption to the clinic.

The American tourist is believed to have paid about $50,000 (£32,000) to go on the hunt in Zimbabwe.

Prosecutors in Zimbabwe have charged the hunter who supervised Mr Palmer's outing, Theo Bronkhorst, for killing a lion not authorised to be hunted. If convicted, he faces up to 15 years in prison.

The country's safari organisation also said the way in which Cecil was lured out of a national park was unethical and possibly illegal.

A second suspect, farm owner Honest Ndlovu, was also arrested but is yet to be charged.

Lions in Africa 35,000 Max estimated lion population 12,000 Max lion population in southern Africa 665 Approx number of 'trophy' lions killed for export from Africa per year

49 Lion 'trophies' exported from Zimbabwe in 2013

0.29% Contribution to GDP of Zimbabwe from trophy hunting

17% Of Zimbabwe's land given to trophy hunting AFP

An international hunting organisation suspended the memberships of both Mr Palmer and Mr Bronkhorst on Thursday, saying it wanted a "full and thorough investigation" into the lion's death.

Safari Club International, which promotes big-game hunting worldwide, said "those who intentionally take wildlife illegally should be prosecuted and punished to the maximum extent allowed by law".

Cecil, who was a major tourist attraction at Zimbabwe's largest game reserve in Hwange National Park, was being monitored by UK-based Oxford University as part of a conservation programme.

The animal is believed to have died on 1 July, but the carcass was not discovered until a few days later. It had been skinned and beheaded.

Analysis: The view from Africa - BBC Monitoring

Cecil's killing has attracted little media attention inside Africa. What little comment there was came in the form of derisive editorials in Zimbabwean state media.

"Not since Simba, of The Lion King fame, has a lion captured the world's imagination in this way," Alex Magaisa wrote in the Zimbabwean Herald newspaper. While tragic, the lion's death has not inflamed local passions because it is "far removed from the lived realities of most of the local people," he added, saying that tourism and hunting in Zimbabwe are "mired in elitism".

The writer said neither he nor his family had heard of Cecil the lion before it was killed.

Kennedy Mavhumashava struck a similar note in the Zimbabwe Chronicle and invoked the history of Western colonialism: "Many believe the lion was named after Cecil John Rhodes, the celebrated forerunner of British colonialism in Southern Africa, explaining the saturation coverage on the demise of his namesake."