Timothy Heaphy, a U.S. Attorney testifying in favor of tightening gun control laws, admitted that he couldn’t explain why some places with tight gun laws have high crime while others have low crime.

“I can’t say that I know what causes gun violence,” Heaphy told the Senate Judiciary Subcommittee on the Constitution, Civil Rights and Human Rights as he called for lawmakers to find the “sweet spot” between Second Amendment rights and the need for public safety. “It’s very difficult to isolate one cause and tie it very specifically to our crime rate,” he said.

Heaphy was responding to Sen. Mazie Hirono, D-Hawaii, who asked him if there are any studies about whether violence goes down when there a looser gun laws. Hirono also mentioned that Chicago and Washington, D.C., have strict gun laws but high crime, whereas Hawaii has strict gun laws and low crime.