The missile that downed Malaysia Airlines Flight 17 appears to have been launched from inside Ukraine, according to sources, including a senior U.S. official with knowledge of a preliminary classified intelligence report.

"The signature of the missile shows that it was fired from inside Ukraine," the source said.

Although both the Ukraine and the Russian separatists and their backers in Moscow are blaming each other, the U.S. intelligence community is leaning toward the conclusion that it was fired by separatists

"[We] can't conclusively say it was separatists but some indicators certainly point that direction," the senior U.S. official told Fox News.

Fox News has also learned that the FBI is preparing to deploy at least one FBI personnel to Ukraine, though it is not known how he or she might assist in the investigation.

The plane went down inside Ukraine, about 30 miles from the Russian border, with 298 aboard Thursday. The Boeing 777 was bound for Kuala Lumpur from Amsterdam when it was shot down.

Separatist groups reportedly blocked Ukrainian officials from the scene following the crash, and later said the "black box," or flight data recorder, had been sent to Moscow.

Separatists were still considering whether to give international crash investigators access to the sprawling crash site. Any investigators would need specific permission from the rebel leadership before they could safely film or take photos at the scene.

The White House issued a statement late Thursday, pressing for a "full, credible, and unimpeded international investigation," adding that it was "vital" that no evidence be tampered with at the crash scene.