US officials have begun an investigation into the killing of a lion in Zimbabwe but say they have been unable to reach the American involved, BBC reported Thursday
The American tourist and a dentist Walter Palmer is believed to have paid about $50,000 (£32,000) to go on the hunt in Zimbabwe.
According to reports, 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.
BBC says 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 reportedly 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. 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.
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.
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.