U.S. officials believe that the chemical weapon used on the Syrian town of Khan Sheikhoun Monday was indeed a nerve agent, quite possibly sarin.

Officials say the manner of the delivery described by victims as well as its timing suggests the use of a nerve agent, as do the symptoms of those affected. Specifically, attackers deliver nerve agents like sarin early in the morning to maximize their impact. The time between early evening and early morning is considered ideal for the dispersal of nerve agents, in that the air temperature is coldest near the ground and there is generally less air turbulence, permitting the agent to "stick" rather than quickly dissipate.

PHOTOS: Rescuers Treat Dozens in Syria Chemical Attack

Khan Sheikhoun was hit by explosions and a chemical agent at 6:30 a.m.

The World Health Organization said Tuesday symptoms seen in the victims are "consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents." Activists say 100 people are dead, and relief agencies report 400 injuries.

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A U.S. official familiar with the intelligence reporting agreed, suggesting "some sort of nerve agent" had been used, adding that sarin was used in an attack in August 2013 that killed as many as 1,700 Syrian civilians in the town of Ghouta. In that case, the agent was delivered by short-range rockets again early in the morning. A second U.S. official also familiar with the intelligence told NBC News not to rule out a similar method — either rockets or artillery — in Khan Sheikhoun. People on the ground reported aircraft nearby at the time of the explosion.

UN observers said the chemical had come from the Assad regime’s stockpile, and the Arab League and the European Union said the attack was carried out by the Assad regime. The Assad regime and Russia, its principal ally, blamed rebels.

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Not long after the Ghouta attack, a U.S. and Russian-brokered agreement called for Syria to eliminate its chemical weapons stocks, including sarin, and as one U.S. official said, "it was largely successful." Vast stocks were shipped out of Syria for destruction throughout 2013 and 2014, but the U.S. intelligence community has long believed, as the official added, that "in the realm of likelihood, something was left over."

"Assad is not a trustworthy interlocutor," he added.

In 2016, the United Nations officially blamed the Syrian government for at least three chemical attacks, including a gas attack via aircraft in Qmenas,Syria in 2015.

The Russian government blamed Monday’s mass casualty event on a leak from a rebel chemical weapons cache that had been hit by a Syrian government airstrike.

The Organization for the Prohibition of Chemical Weapons (OPCW), charged with enforcing the Chemical Weapons Convention that has been adopted by nearly 200 countries, said Monday its Fact Finding Mission (FFM) is "in the process of gathering and analysing information from all available sources. The FFM will report its findings to the OPCW’s Executive Council and States Parties to the Chemical Weapons Convention."

However, the OPCW provided no timeframe. Historically, the FFM has been hamstrung by its inability to get access to the sites of suspected attacks.

Since the 2013 agreement to eliminate the Syrian stockpile, Assad's forces have primarily used chlorine in its attacks on supposed "terrorist targets." The use of chlorine, while not on the banned list of chemical agents, is considered a violation of the Chemical Weapons Convention, which bans the use of any chemical as a weapon.