The Human Rights Watch (HRW) announced on Tuesday that evidence "strongly" implies the use of chemical weapons by the Syrian government in the Aug. 21 attack that killed hundreds of people, according to CBS.

Because the international rights organization was unable to travel to Ghouta -- the suburb that was attacked -- to collect hard evidence, they analyzed documents from the attack and consulted with an expert on the detection and effects of chemical warfare agents. 

"This evidence strongly suggests that Syrian government troops launched rockets carrying chemical warheads into the Damascus suburbs that terrible morning," said Peter Bouckaert, HRW's emergencies director. 

The group said the attack was caused by fired rockets with warheads, "likely" containing sarin. They also used "the type of rockets and launchers" used in last month's attack during their investigation. In addition, the group studied documented medical symptoms of victims -- some of which were retrieved from YouTube after local activists posted them online. 

"Two separate surface-to-surface rocket systems believed to be associated with the delivery of chemical agents were identified. The first type of rocket, found at the site of the Eastern Ghouta attacks, is a 330mm rocket that appears to have a warhead designed to be loaded with and deliver a large payload of liquid chemical agent. The second type, found in the Western Ghouta attack, is a Soviet-produced 140mm rocket that, according to reference guides, has the ability to be armed with one of three possible warheads, including one specifically designed to carry and deliver 2.2 kilograms of Sarin," the report read.

Bouckaert added the victims "provide telltale evidence about the weapon systems used."  

Syrian President Bashar Assad continues to deny the claims of his government using chemical weapons and blames it on the rebels. In an interview with Charlie Rose, he said the U.S. should expect retaliation following any strike on Syria, although "not necessarily from the government."