(CNN) -- Until authorities know what happened to Malaysia Airlines Flight 370, they'll look for clues in the histories of everyone on board.
The cruel reality is that every one of the 239 people on board is both a possible victim and a possible suspect -- until proven otherwise.
Already, some passengers and the pilots have fallen under increased scrutiny, and more are likely to come into focus as the search for answers continues.
"You have to look at everybody that got onto that plane," Bill Gavin, former assistant director of the FBI in New York, told CNN's "The Lead with Jake Tapper" on Monday.
"You can start peeling the onion there by eliminating some of the people immediately -- you know, like children, and maybe very elderly people, or infirmed people. You might be able to eliminate those folks.
"But, by the same token, you really have to look through the whole category of people that are on the plane," he said.
Here's what we know so far about some of the people investigators want to know more about:
Pilot: Zaharie Ahmad Shah
Malaysia's Prime Minister has said that somebody deliberately steered the plane off course. That means the pilots have become one obvious area of focus.
On Saturday, Malaysian police searched Zaharie's home. The 53-year-old pilot lives in an upscale, gated community in Shah Alam, outside Malaysia's capital, Kuala Lumpur.
Malaysian police said Sunday they were still investigating a flight simulator seized from that house.
It's somewhat common among aviation enthusiasts to use online flight simulator programs to replicate various situations.
Peter Chong, a friend of Zaharie's, said it's unfair to imply the pilot had anything to do with what happened to the plane.
He said he'd been to Zaharie's house and tried out the flight simulator.
"It's a reflection of his love for people," Chong said, "because he wants to share the joy of flying with his friends."
Shah joined Malaysia Airlines in 1981 and has more than 18,000 flying hours.
Co-pilot: Fariq Ab Hamid
Fariq Ab Hamid, 27, started at the airline in 2007 and has 2,763 flying hours.
Two vans were loaded with small bags, similar to shopping bags, at the home of the co-pilot, according to a CNN crew who observed activities at the residence.
It was unclear whether the bags were taken from the home, and police made no comment about their activities there.
U.S. intelligence officials are leaning toward the theory that "those in the cockpit" -- the captain and co-pilot -- were responsible for the mysterious disappearance, a U.S. official with direct knowledge of the latest thinking has told CNN.
The official emphasized no final conclusions have been drawn and all the internal intelligence discussions are based on preliminary assessments of what is known to date.
Acting Malaysian Transportation Minister Hishammuddin Hussein has told reporters the pilots didn't request to work together.
Passenger: Mohammed Khairul Amri Selamat
The 29-year-old Malaysian civil aviation engineer works for a private jet charter company.
Although police are investigating all passengers and crew, he is likely to be of particular interest because of his aviation knowledge.
"I am confident that he is not involved," his father said on Monday. "They're welcome to investigate me and my family."
The bottom line, investigators say, is that whoever flew the plane off course for hours appeared to know what they were doing.
They are looking into the backgrounds of the passengers to see whether any of them were trained pilots.
"There are still a few countries who have yet to respond to our request for a background check," said Khalid Abu Bakar, inspector general of the Royal Malaysian Police Force. "But there are a few ... foreign intelligence agencies who have cleared all the(ir) passengers."
Passengers: Pouri Nourmohammadi and Delavar Seyed Mohammad Reza
In the first few days after the plane went missing, investigators focused a lot on two passengers who boarded the plane using stolen passports.
Authorities have since identified them as Nourmohammadi, 18, and Reza, 29, both Iranians.
The men entered Malaysia on February 28 using valid Iranian passports, according to Interpol.
Malaysian police believe Nourmohammadi was trying to emigrate to Germany using a stolen Austrian passport. His mother contacted police after her son didn't arrive in Frankfurt as expected.
Malaysian investigators say neither of the men has any apparent connection to terrorist organizations.
Stolen passports don't necessarily indicate terrorism. In fact, passengers flew without having their travel documents checked against Interpol's lost-and-stolen passport database more than a billion times in 2013, according to the international police organization.