The U.S. State Department said on Thursday that it has “credible information” Russia and Syria are working together to keep inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW) from visiting the site of the alleged chemical attack in Douma.

“We have credible information that indicates that Russian officials are working with the Syrian regime to deny and to delay these inspectors from gaining access to Douma,” State Department spokeswoman Heather Nauert said at a press briefing.

“Russian officials have worked with the Syrian regime to sanitize the locations of the suspected attacks and remove incriminating evidence of chemical weapons use,” Nauert charged.

Although Syrian state media reported that OPCW inspectors arrived at Douma on Tuesday, that proved to be true only in the most narrow technical sense; they were turned away from the attack sites by gunfire, explosives, and an angry mob.

The inspectors are presently awaiting approval from U.N. security before attempting another visit, with no announced timetable for when that might happen. The OPCW has made it clear the Russian military and Syrian government are responsible for the safety of the inspectors.