(CNN) -- The United States has evidence that the chemical weapon sarin has been used in Syria on a small scale, Defense Secretary Chuck Hagel said Thursday.
But numerous questions remain about the origins of the chemical and what impact its apparent use could have on the ongoing Syrian civil war and international involvement in it.
When asked if the intelligence community's conclusion pushed the situation across President Barack Obama's "red line" that could potentially trigger more U.S. involvement in the Syrian civil war, Hagel said it's too soon to say.
"We need all the facts. We need all the information," he said. "What I've just given you is what our intelligence community has said they know. As I also said, they are still assessing and they are still looking at what happened, who was responsible and the other specifics that we'll need."
In a letter sent to lawmakers before Hagel's announcement, the White House said that intelligence analysts have concluded "with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."
In the letter, signed by White House legislative affairs office Director Miguel Rodriguez, the White House said the "chain of custody" of the chemicals was not clear and that intelligence analysts could not confirm the circumstances under which the sarin was used, including the role of Syrian President Bashar al-Assad's regime.
But, the letter said, "we do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime."
The Syrian government has been battling a rebellion for more than two years, bringing international condemnation of the regime and pleas for greater international assistance.
The United Nations estimated in February that more than 70,000 people had died since the conflict began.
The administration is "pressing for a comprehensive United Nations investigation that can credibly evaluate the evidence and establish what took place," the White House letter said.
Sen. John McCain, one of the lawmakers who received the letter, said the use of chemical weapons was only a matter of time.
"It should not surprise us," he told CNN's Wolf Blitzer on Thursday. "This guy will do whatever is necessary to assure his position of power," he said of al-Assad.
Earlier, McCain urged the administration to work for the establishment of a safe zone for Syrian rebels.
"Everything that the non-interventionists said would happen in Syria if we intervened has happened," he said. "The jihadists are on the ascendency, there is chemical weapons being used, the massacres continue."
"The president of the United States said that if Bashar Assad used chemical weapons that it would be a game changer, that it would cross a red line," McCain said. "I think it is pretty obvious that red line has been crossed."
The announcement comes a few days after an Israeli intelligence official said Damascus was using weapons banned under international law against its own people in the country's civil war.
On Wednesday, Israeli President Shimon Peres told CNN that he expected the United States to fall in line with its estimate on chemical weapons use in Syria.
"I think the United States and us and others will do whatever we can to meet this very dangerous weapon," he said. "The sooner the better."
Syria has said it is rebels who have used chemical weapons.
Sarin gas is an odorless nerve agent that can cause convulsions, paralysis and respiratory failure. Nerve agents are the most toxic and rapidly acting of the known chemical warfare agents, according to the Centers for Disease Control and Prevention.
Sarin quickly evaporates from liquid to vapor form to disperse into the environment. It also mixes easily with water and can poison a water supply.
CNN's Joe Sterling, Sara Sidner and Jessica Yellin contributed to this report.