The dentist who killed Cecil, a popular lion in Zimbabwe, has apologized to his patients. 

Walter J. Palmer, in the letter, told patients he had no idea his hunt was illegal. He said he had paid fees to obtain the proper permits. 

Palmer has closed his practice and gone into hiding.

As controversy over Cecil's killing continues, a memorial for the lion has grown outside of Palmer's office.

Several petitions have surfaced online about Palmer, including one calling on President Obama to extradite Palmer to Zimbabwe. 

On Thursday, the U.S. Fish & Wildlife Service said it was trying to reach the dentist as it continues to investigate the circumstances surrounding Cecil's death.

(Copyright (c) 2015 Sunbeam Television. The Associated Press contributed to this report. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)

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