The US Fish and Wildlife Service (FWS) has launched an investigation into the killing of Cecil the lion in Zimbabwe but say they have been unable to reach dentist Walter Palmer.

The FWS said it was “deeply concerned” about the “tragic” death of one of Africa’s most famous lions.

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

Walter Palmer says he thought the hunt was legal but two Zimbabwean men have been arrested over Cecil’s death.

The dental practice Walter Palmer 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 July 29, carrying placards saying “Justice for Cecil”, “Trophy hunters are cowards” and “Prosecute poachers”.

On July 30, the White House said it would review a public petition to extradite Walter Palmer after more than 100,000 signed it.

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

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 Walter 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 Walter Palmer is currently unknown, but he is thought to have returned to the US after Cecil was killed on July 1st.

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

He is believed to have paid about $50,000 to go on the hunt in Zimbabwe.

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

Zimbabwe’s safari organization 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.

An international hunting organization suspended the memberships of both Walter Palmer and Theo Bronkhorst on July 30, 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 Oxford University as part of a conservation program.

The lion is believed to have died on July 1st, but the carcass was not discovered until a few days later.