US officials have launched an investigation into the killing of Cecil the 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. It was gathering facts about the issue and would assist Zimbabwe but efforts to reach Minnesota dentist Walter Palmer had been unsuccessful.
Mr Palmer said he thought the hunt was legal but two Zimbabwean men have been arrested over the popular lion's death.
The dental practice he runs has been closed since he was named as the tourist who shot Cecil, Zimbabwe's most famous lion.
Protesters have gathered outside the building carrying placards saying "Justice for Cecil", "Trophy hunters are cowards" and "Killer".
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.
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.
Describing himself as a "life-long hunter" he said he rarely discussed his passion with patients "because it can be a divisive and emotionally charged topic".
The American tourist is believed to have paid about $US50,000 to go on the hunt in Zimbabwe.
More than 600 'trophy' lions are believed to be killed each year in Africa, and 49 were exported from Zimbabwe in 2013. The lion population on the continent is estimated at 35,000.