Executions in Iran surged to nearly 1,000 in 2015, a United Nations investigator said on Thursday, the highest level in more than a quarter-century.

The investigator, Ahmed Shaheed, the special rapporteur for human rights in Iran, said in a report to the organization’s Human Rights Council that at least 966 people were put to death in the country last year, roughly double the number executed in 2010 and 10 times as many as were executed in 2005.

Iran has been one of the world’s leading users of the death penalty, along with China and Saudi Arabia.

According to annual figures on capital punishment compiled by Amnesty International, the 2015 figure for Iran is the highest since 1989, when more than 1,500 people were executed. Most executions in Iran are by hanging, with a majority of the condemned having been convicted of drug-related offenses.