The FBI is sending agents and technical experts to assist a team probing the disappearances of a Malaysia Airlines jet that had several Americans aboard, US media reported on Saturday.
US officials told The Los Angeles Times that they are trying to determine whether there was any terror link to what caused Flight 370 to go missing as it carried 239 people from Kuala Lumpur toward Beijing.
The fact that at least three of the passengers are believed to be Americans "gives us entree" to the case," a top federal law enforcement official told the newspaper.
"But so far, what happened is a mystery."
A US official told CNN that FBI agents stationed at the American Embassy in Kuala Lumpur, including an FBI legal attache, were monitoring the situation closely.
Asked to confirm the reports, an FBI spokesman only replied: "We are ready to assist if needed."
Although two passengers appeared to have been using stolen EU passports, "there is no indication this is a terrorist attack; stolen passports are certainly not indicative of a terrorist attack," a senior counterterrorism official told the Times.
The official stressed there was "no evidence" of terrorism thus far.
According to the federal law enforcement official, FBI personnel will help review video of the Kuala Lumpur airport for images of passengers at the ticket counter, security sections and the boarding area. The agents would then use counterterrorism technology to find any possible matches with known members of Al-Qaeda or other terror groups.
And the US National Transportation Safety Board may also join the investigation "because the jet was built by Boeing in this country," the law enforcement official said.
The stolen passports used by two passengers on the plane are believed to have come from an Italian and a an Austrian.
"Just because they were stolen doesn't mean the travelers were terrorists," a Department of Homeland Security official said. "They could have been nothing more than thieves. Or they could have simply bought the passports on the black market."