Seven people have been arrested in Britain on suspicion of financing terrorism in Somalia by smuggling a leaf that can produce a mild high into the United States, officials said Tuesday.

Scotland Yard said the group was arrested as part of an operation that involved Homeland Security Investigations, the investigative branch of U.S. Immigrations and Customs Enforcement.

The operation was investigating a network suspected of illegally exporting a leaf known as khat from the U.K., where it is legal, to the U.S. and Canada, where it is a controlled substance, Scotland Yard said.

"Law enforcement had developed leads, in the U.K. and U.S., that khat was being transshipped through the U.K., then illegally smuggled into the United States," said Ross Feinstein, a spokesman for U.S. Immigration and Customs Enforcement. "The proceeds generated by this illegal activity (were) then transferred back to Somalia."