When the first pieces of information circumvented a near-total blackout during Iran’s protests last month, rights defender Mahmood Amiry-Moghaddam was already ready to say the scale of the crackdown was “unbelievable.”

“We have never experienced something like this,” said the director of the Norway-based nongovernmental organization Iran Human Rights (IHR), which has been tracking rights violations in the Islamic republic for some two decades.

As a fragmented picture of the anti-government protests that peaked in early January emerged, IHR and other NGOs set out to verify the reports of thousands of deaths — painstaking work they are still undertaking weeks later.