Information emerging from Iran suggests the government’s crackdown on more than two weeks of anti-government protests may have been far deadlier than previously reported.

Sources speaking to CBS News, including one inside Iran, claim activist groups believe at least 12,000 people—and possibly up to 20,000—have been killed, though these figures cannot be independently verified.

Britain’s foreign secretary said the U.K. believes deaths exceed 2,000 and may ultimately be significantly higher.

Verifying the toll has been extremely difficult due to widespread internet and phone shutdowns, intimidation of hospital staff, and lack of official transparency from Iranian authorities, CBS News has reported.

While Iran’s government has suggested about 2,000 deaths, independent media and verified video evidence from a Tehran morgue indicate hundreds of bodies with severe injuries, pointing to a potentially much larger scale of violence.