[Capitán Peligroso]

Two of the people on the plane were traveling on stolen passports, the proper owners of which have been located in their home countries.

There are criminal reasons other than terrorism (drugs, etc.) why people might travel on stolen passports, but according to the article, it's quite rare that two of them are on one plane. In any case, the airline and airport authorities had access to the Interpol database that would have identified the passports as stolen.

Quote:

Fuentes said that Interpol, the international law enforcement agency, maintains an extensive database of lost and stolen passports. The use of the stolen passports by passengers on the Malaysia Airlines flight should have been checked against that database by airline officials, he said.
"Now the question here becomes, did the authorities in Malaysia, the airport in Kuala Lumpur, did they make inquiry of that database," Fuentes said. "Is that system set up to make an automatic inquiry if someone is using a previously reported stolen document? That should come up right away if they check that database. Not every country that belongs to Interpol automatically does that."

There is a lot of fodder for speculation as more information comes in.