SACRAMENTO, United States, Sept. 11 (Xinhua) -- U.S. investigators on Thursday expanded the search for the gunman who killed political activist Charlie Kirk and offered a reward of 100,000 U.S. dollars for information leading to the arrest of the person responsible for Kirk's shooting.

The Federal Bureau of Investigation has released two images of the "person of interest."

Kirk, 31, was shot in the neck as he was speaking to a large crowd at an outdoor "Prove Me Wrong" debate at Utah Valley University around midday Wednesday.