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(CNN) – Malaysia Airlines Flight 370 took off from Kuala Lumpur shortly before 1 a.m. Saturday (1 p.m. Friday ET). The Boeing 777-200ER, carrying 227 passengers and 12 crew members, went missing while flying to Beijing. Days later, there is still no sign of the missing plane.
"The FBI has to presume that it's terrorism, and do the types of things to conduct a preliminary investigation," said former FBI Assistant Director James Kallstrom conducted the criminal investigation into TWA flight 800, which exploded shortly after take-off from JFK in 1996.
"It's very, very active. It's complex now. You're talking about two or three different countries. They don't know where the plane is. The radar coverage is sketchy, at best. And they are looking in the water but they should also probably look on land in Vietnam," said Kallstrom.
"There're areas of Vietnam where planes go in, and people wouldn't even know it," said the former FBI official.
The similarities between TWA 800 and Malaysia Airlines Flight 370 are, says Kallstrom, "alike in as much as it's a tragedy, and it's likely over water."
If the missing flight did crash into the ocean, "and if there was a catastrophic break-up at altitude or if the pilots flew it into the ocean themselves, depending on the depth of that water, you're going to have a very, very difficult time," said Kallstrom.
"130 feet was a big problem for us. We had all of the resources of the U.S. Navy helping us. I don't know what they'll do over there if it's 1,000 feet deep," said Kallstrom.
For more of our interview with former FBI official James Kallstrom, check out the video above.