Of the civilians held there, nearly 128,000 never emerged, and nearly 14,000 were “killed under torture,” an independent monitoring group found.

The Syrian government denies the existence of systematic abuse, but recently discovered memos show that officials who report to Mr. al-Assad ordered mass detentions and knew of atrocities.

How we know: Over seven years, The New York Times interviewed dozens of survivors and relatives of dead and missing detainees, examined hundreds of pages of witness testimony, and reviewed government documents detailing prison deaths and crackdowns on dissent.

Pompeo prepares to meet Putin

Mike Pompeo is scheduled to make his first trip to Russia as secretary of state this week. On his agenda: A discussion with President Vladimir Putin about disinformation efforts before European elections this month.

Investigators say Moscow used many of the same digital tactics to interfere in the 2016 U.S. presidential vote. But a former F.B.I. analyst said that the Kremlin’s ultimate goal extends beyond any one election: “It is to constantly divide, increase distrust and undermine our faith in institutions and democracy itself.”