DOUMA, Syria — Syrian and Russian authorities prevented independent investigators from going to the scene of a suspected chemical attack, the head of the chemical watchdog group said Monday, blocking international efforts to establish what happened and who was to blame.

The U.S. and France say they have evidence that poison gas was used in the April 7 attack in the opposition-held town of Douma, killing dozens of people, and that Syrian President Bashar Assad’s military was behind it.

But they have made none of that evidence public, even after they, along with Britain, bombarded sites they said were linked to Syria’s chemical weapons program.

Syria and its ally Russia deny any chemical attack took place, and Russian officials went even further, accusing Britain of staging a “fake” chemical attack. British Prime Minister Theresa May accused the two countries – whose forces now control the town east of Damascus – of trying to cover up evidence.

The lack of access to Douma by inspectors from the watchdog group, the Organization for the Prohibition of Chemical Weapons, has left unanswered questions about the attack.

Director-General Ahmet Uzumcu said Syrian and Russian officials cited “pending security issues” in keeping its inspectors from reaching Douma.

“The team has not yet deployed to Douma,” Uzumcu told an executive council meeting of the OPCW in The Hague.

Instead, Syrian authorities offered them 22 people to interview as witnesses, he said, adding that he hoped “all necessary arrangements will be made … to allow the team to deploy to Douma as soon as possible.”

Russian military police were ready to help protect the OPCW experts on their visit to Douma, said Maj. Gen. Yuri Yevtushenko of the Russian military’s Reconciliation Center in Syria. Igor Kirillov, a Russian chemical weapons protection expert in The Hague, said the team is due to visit the site Wednesday.

Earlier Monday, Russian Deputy Foreign Minister Sergei Ryabkov said the inspectors could not go to the site because they needed approval from the U.N. Department for Safety and Security. He denied that Russia was hampering the mission and suggested the approval was held up because of the Western airstrikes.

“As far as I understand, what is hampering a speedy resolution of this problem is the consequences of the illegal, unlawful military action that Great Britain and other countries conducted on Saturday,” he said.

However, U.N. spokesman Stephane Dujarric said the United Nations has “provided the necessary clearances for the OPCW team to go about its work in Douma. We have not denied the team any request for it to go to Douma.”

At least 40 people are believed to have died April 7 in Douma, which until Saturday was the last rebel-held town near the capital and the target of a government offensive in February and March that killed hundreds and displaced tens of thousands. Hours after the alleged chemical attack, the rebel faction that controlled the town, the Army of Islam, relented and was evacuated along with thousands of residents.

The Associated Press, during a government-organized visit Monday to Douma, spoke to survivors and witnesses who described being hit by gas. Several said a strange smell started spreading and people screamed, “It’s chlorine! It’s chlorine!”

The AP visited a two-room underground shelter where Khaled Mahmoud Nuseir said 47 people were killed, including his pregnant wife and two daughters, 18-month-old Qamar and 21/2-year-old Nour. A strange smell lingered, nine days after the attack.

Nuseir, 25, said he ran from the shelter to a nearby clinic and fainted. After he was revived, he returned to the shelter and found his wife and daughters dead, with foam coming from their mouths.

He and two other residents accused the rebel Army of Islam of carrying out the attack. As they spoke, government troops were not far away but out of earshot. Nuseir said a gas cylinder was found leaking the poison gas, adding that he didn’t think it was dropped from the air because it still looked intact.

Separately, the AP spoke to a medic who was among those who later were evacuated to northern Syria. Ahmed Abed al-Nafaa said helicopters were flying before the attack and when he reached the site, people were screaming “chlorine.” He said he tried to enter the shelter but was overcome by a strong smell of chlorine and his comrades pulled him out.

The accounts contradict what the Syrian government and Russia have reported: that there was no gas attack in Douma.