One of the relatives of Chinese passengers on Malaysia Airlines Flight 370 talks on his mobile phone as he touches a board covered with written messages to the people who were on the plane. Associated Press

Hundreds of interviews and background checks have turned up nothing to incriminate the passengers and crew in the disappearance of a Malaysia Airlines 3786.KU -2.38% Malaysian Airline System Bhd Malaysia RM0.20 -0.01 -2.38% March 28, 2014 4:59 pm Volume : 110.88M P/E Ratio N/A Market Cap RM3.43 Billion Dividend Yield N/A Rev. per Employee N/A 0.2100.2050.20010a11a12p1p2p3p4p 03/30/14 Australian Maritime Safety Aut... 03/30/14 Malaysia Digs Deeper Into Airp... 03/29/14 Sunday Talk Preview: NSA, Ukra... More quote details and news » jet, authorities say, prompting officials to look again and more thoroughly in a bid to identify possible suspects while scrutinizing airport security procedures.

The multinational investigative effort has left investigators with no clear leads to why the plane ended up thousands of miles off course, with satellite and radar data analysis leading searchers to zero in on the Indian Ocean to hunt for wreckage but nothing so concrete about what happened to get it there.

The captain of an Australian P3 Orion aircraft that had been searching for objects related to missing Malaysia Airlines Fight MH370 said on Saturday they didn't find anything of significance. Photo: AP

Lost at Sea: Profiles From Flight 370

"We cannot zero in on any faults by passengers or crew members so we are focusing on getting into value-added information in order to strengthen our investigative findings," Malaysian Home Minister Ahmad Zahid Hamidi told reporters Saturday in Kuala Lumpur. He didn't elaborate.

Mr. Zahid also said that officials were re-examining airport security procedures. "We are revisiting our standard operating procedures," he said, "especially the protocol of our security at our entry points; especially at our Kuala Lumpur International Airport."

Malaysian investigators believe Flight 370 deviated from its original flight path on March 8 from Kuala Lumpur to Beijing due to what Prime Minister Najib Razak has termed "deliberate action." They have been careful not to rule out any possible cause for the plane's disappearance and say they are focusing on hijacking, sabotage and personal or psychological problems, without elaborating.

The country's police chief, Khalid Abu Bakar, said earlier last week that all passengers had been cleared by their respective countries and that Malaysia was looking again at the crew. He didn't provide details.

"It's more likely they found nothing of concern for any of those people, and if there were one or two people who were of concern, I think they would pop out more quickly unless they were a complete unknown," said Justin Gosling, an independent law enforcement consultant and former criminal intelligence officer at Interpol.

Indeed, two passengers who boarded Flight 370 and were traveling on stolen passports were identified by investigators within three days of the flight's disappearance when their records were checked against an Interpol database. Pouria Nourmohammadi Mehrdad, 19, and Delavar Seyed Mohammadreza, 30, were both from Iran. Neither was found to have any militant links. The younger man appears to have been trying to reunite with his mother in Germany.

Malaysia doesn't consult Interpol's database on a routine basis during the passenger boarding process. Mr. Zahid, the home minister, suggested in Parliament last week that the database would be too slow to work with Malaysian systems, a statement that drew an unusually sharp rebuke from Interpol.

"Interpol has no idea why Malaysia's home minister chooses to attack Interpol instead of learning from this tragedy," the agency said in a statement, adding that the database takes just seconds to access and that the U.S., one of the member countries using it, checks it 230 million times a year.

Malaysian officials turned to the 13 other countries with passengers on Flight 370 in the early days of the investigation to ask them for background checks on their own citizens. The country's police chief said that more than 100 interviews had been completed as part of their own investigation.

"It would be quite challenging to do very thorough checks," said Mr. Gosling, since some countries do not have nationalized databases to share information between relevant departments. More thorough checks could involve looking at travel habits of the passengers but those would likely involve non-law enforcement networks, such as immigration databases, he said. "And then you also have to join the dots from all the different countries."

Indonesia said it had cleared all seven of the Indonesian nationals on Flight 370. "We did a thorough check on them using international standards. We check their background, their track records. We found that they are not linked to any forbidden organizations," said police spokesman Agus Rikwanto.

China, which had 153 nationals on board Flight 370, has also cleared its own passengers. Chinese officials didn't respond to requests to comment about the process.

Malaysian investigators have focused their attention on passengers and crew with the experience to have cut off the plane's communications with air traffic control and steered it off course. The plane's pilots, Capt. Zaharie Ahmad Shah and co-pilot Fariq Abdul Hamid, have been subject to most scrutiny, but investigators haven't turned up anything suspicious relating to either man.

A homemade flight simulator belonging to Mr. Zaharie was seized by police and analyzed both in Malaysia and then again in the U.S. by the Federal Bureau of Investigation. Neither analysis turned up anything suspicious.

"Malaysian police can effectively investigate our own citizens," said P. Sundramoorthy, head of criminology research at Universiti Sains Malaysia. But he added that diplomatic relations can be a sticky business. "You must understand that the sovereignty of a nation goes beyond all. Just because we are investigating here, it does not compel the other countries involved in this to provide'' comprehensive information.

—I-Made Sentana, Celine Fernandez and David Pearson contributed to this article.