Cecil wasn't the first lion illegally hunted by an American doctor this year, authorities in Zimbabwe said on Sunday.
A Pennsylvania cancer doctor and bow hunter Jan Seski is accused of killing another lion in April, wildlife officials allege.
Officials say Seski's local-based guide "hunted without a quota and a permit."
The guide was arrested and officials say he's cooperating. Seski did not answer calls from NBC News.
Seski has not been accused of committing a crime and it is unclear whether Zimbabwe authorities will seek to extradite him.