Malaysian authorities have cleared all 227 passengers on board missing Malaysia Airlines Flight MH370 of any suspicion, but the plane's cabin crew is still being investigated, local news reports said Wednesday.

The police said they had cleared some passengers who were initially suspected of possibly being part of a hijacking or sabotage of the plane, which went missing on March 8 and whose fate remains unknown. Local authorities also cleared the plane's passengers of having any personal or psychological problems that could have been connected to the flight's disappearance, but said they are also investigating the cargo and the food served on the plane to check for the possibility of poisoning, Associated Press reported.

"Investigations may go on and on and on. We have to clear every little thing," Inspector General Khalid Abu Bakar reportedly said, according to AP, adding: "At the end of the investigations, we may not even know the real cause. We may not ever know the reason for this incident."

Khalid said that the police have recorded statements from 170 people, including relatives of the passengers and pilots, regarding the mysterious disappearance and that the search for Flight MH370 was now classified as a criminal investigation.

The so-far-fruitless search for the plane, which disappeared nearly a month ago, has been joined by private jets, which are assisting international agencies from around the world. On Wednesday, close to 10 planes and nine ships from six countries searched a portion of the southern Indian Ocean for the black box recorder of the plane, which if found could shed light on the aircraft's whereabouts.

"We don't have a precise aircraft location for six hours before the aircraft went into the water somewhere," Angus Houston, a retired defense official who is overseeing Australia's role in the search, reportedly said in an interview with the Australian Broadcasting Corporation on Wednesday. "The reality is it's the most complex and challenging search and rescue operation, or search and recovery operation now, that I've ever seen."