(CNN) -- American investigators reviewing a hard drive belonging to the captain of missing Malaysia Airlines Flight 370 have found that there were deletions of information even closer to the final flight than first indicated by Malaysian officials, U.S. law enforcement officials tell CNN.
The forensic search of the computer files by government experts found files were removed even after February 3, the date Malaysian authorities have cited for when some data was cleared from the drive of the captain.
Investigators are examining the contents of drives belonging to both pilots. Copies of the hard drives are at the FBI's forensics lab facilities in Quantico, Virginia, and one is being analyzed with the assistance of consultants the FBI uses to help analyze such electronic data.
It's not clear why Malaysian officials cited the February 3 date and if they knew of the other deletions. The type of software used for flight simulation takes up a lot of room in hard drives and investigators believe that could be one reason for deletion of files.
It's possible too that some damage could have been done during the first examinations done by Malaysian investigators.
Law enforcement officials say that they aren't drawing any conclusions about the subsequent deletions, or the earlier ones, just two days into reviewing the hard drive contents, which officials described as a large volume of data. More couldn't be learned about the nature of the deletions, and who made them.
Some senior U.S. counterterrorism officials say that an accident is the leading operative theory as cause for the disappearance of Flight 370. That's because there is no other information indicating foul play.
"Barring other information to suggest otherwise one has to first think this was a tragic accident," a U.S. law enforcement official said. But investigators have not ruled out terrorism or other theories.
Investigators have focused on the pilots because of the lack of any other information. But there isn't proof that they did anything wrong.
U.S. investigators have compiled profiles of the two pilots, based on interviews with friends, neighbors and family members conducted by Malaysian investigators, and on a search of their online activities, U.S. officials say.
Those interviews haven't turned up anything that could suggest any explanations for the plane's disappearance.
U.S. investigators also are concerned about the preservation of evidence in Malaysia.
Malaysian authorities waited six days to search the pilots' homes. This was enough time, U.S. officials believe, for someone who had access to the homes perhaps to have tampered with evidence.
They don't know if there was any tampering but it is a worry because Malaysian officials didn't appear to secure evidence immediately.
U.S. officials say they understand that there are Malaysian legal restrictions and requirements for probable cause before such searches. The difference in procedures is a common issue in international investigations such as this.