This image released April 8, 2018, by the Syrian Civil Defense White Helmets, shows a child receiving oxygen through a respirator following an alleged poison gas attack in the rebel-held town of Douma, near Damascus, Syria. Syrian Civil Defense White Helmets/AP

The head of the world's chemical weapons watchdog said Monday he stands by a report on an alleged attack in Syria despite leaked documents casting doubt on its conclusions. The Wikileaks website published an email over the weekend from a member of the team that investigated the attack in the town of Douma in April 2018 that accused the body of covering up discrepancies.

Russia and its allies have seized on the email and an earlier document which both question the conclusion by the Organization for the Prohibition of Chemical Weapons (OPCW) in March 2019 that chlorine was used in Douma, likely dropped from aircraft.

Britain, France and the United States unleashed missile attacks on three suspected chemical weapons facilities run by President Bashar al-Assad's regime after the attack.

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First responders said 40 people were killed in the Douma chemical attack. OPCW inspectors who went to Syria to investigate the attack said they were delayed for days in accessing Douma by Russian and Syrian authorities, who claimed security would not permit the inspectors to reach the site.

"It is in the nature of any thorough inquiry for individuals in a team to express subjective views," OPCW Director General Fernando Arias said in a speech to the body's annual meeting in The Hague.

"While some of these diverse views continue to circulate in some public discussion forums, I would like to reiterate that I stand by the independent, professional conclusion" of the probe.

Just nine days after the attack on Douma, with the OPCW inspectors still held back by Russian and Syrian officials, CBS News' Seth Doane made it into the town and to the very house where the suspected chemical attack took place.

"All of a sudden some gas spread around us," one neighbor told Doane. "We couldn't breathe, it smelled like chlorine."

Nasr Hanan's brother Hamzeh is one of the people who appeared in videos posted online by activists. Hamzeh was seen lifeless and foaming at the mouth. His brother told CBS News how Hamzeh had tried to wash the chemicals off of himself before succumbing. He also brought CBS News to the roof of the building, where a large missile still rested in a hole it appeared to have created when it struck. That shell, he said, is what delivered the noxious gas that killed his brother.

The leak of the email comes amid tension over the designation of a new OPCW team that will for the first time have a mandate to name the culprits behind chemical attacks in Syria. Russia had opposed granting the team the new responsibility to officially point the finger over such attacks, and Moscow threatened to block next year's budget for the OPCW over it.

The United States, Britain, France and other allied members of the OPCW believe, however, that they have enough support to pass the budget with a large majority at the annual meeting of the group's 193 member states in The Hague, which began on Monday. OPCW members agreed in 2018 to give the U.N.-backed agency the new powers to name culprits of chemical attacks.

Details of the Douma incident have now been passed to the new OPCW team, Arias said.

At the time of the initial Douma probe, the OPCW only had the mandate to say whether or not toxic arms had been used, not to name perpetrators.

The email by an unidentified investigator quoted by Wikileaks expresses the "gravest concern," saying the OPCW report "misrepresents the facts" and contains "unintended bias."

It focuses on the levels of chlorine allegedly found at the site, and whether or not barrels believed to have contained the chemical had been dropped from the air or placed manually. The Russian-backed Syrian regime had complete control of the airspace over Douma and was the only power flying aircraft in the region at the time.

Russia and Syria have alleged that the incident was staged since soon after it took place. Russia even attempted to bolster its case by bringing individuals who it identified as Syrians seen in "staged" videos after the attack to testify to the OPCW at The Hague.

European OPCW members rejected the Russian-Syrian claims outright, and refused to attend the session at OPCW headquarters.

"This obscene masquerade does not come as a surprise from the Syrian government, which has massacred and gassed its own people for the last seven years," France's ambassador to the Netherlands, Philippe Lalliot, said in response.

The OPCW has already launched an internal investigation into the leak of the earlier document by a member of the Douma team raising similar concerns.

Also slated for discussion at this week's OPCW meeting is a proposal to add Novichok nerve agents to the organization's list of officially banned chemical weapons.

Novichok was developed by the Soviet Union during the Cold War. It was used in the attempted assassination last year of Russian spy-turned-double-agent Sergei Skripal in England.

The U.K. accused Russia of the March 2018 attack in the quiet town of Salisbury, which sickened Skripal and his daughter and left one innocent resident dead from exposure to the toxic chemicals.

Russia has continued to deny any involvement, but Britain's allies, including the U.S., expelled Russian diplomats from missions in their countries in solidarity with the United Kingdom.