The explosives, weighing about 100 grams and wrapped in aluminium, were found during a customs check aboard the Boeing 737 at Metz-Nancy airport in eastern France late on Wednesday.
The airline said investigations had so far revealed nothing about the source or destination of the explosives.
Sniffer dogs found the stick hidden between two seats during a check of the aircraft after passengers had disembarked. There was no detonator.
French investigators are pursuing identity checks on the 160 passengers on the flight as well as the aircraft's crew.
The two passengers seated near the explosives were found to be tourists with no criminal records, investigators said.
No-one has yet been detained in connection with the incident, nor has anyone claimed responsibility for placing the explosives on the plane.
'Shoe' explosive
Investigators have speculated that the incident was either a delivery gone wrong or a bombing in progress.
French experts have said the quantity of pentrite found on the plane did not constitute in itself a threat.
Pentrite is a granular powder which can be used alone or in composition with other chemicals such as Semtex.
Semtex was used in the 1988 Lockerbie bombing which killed 270 people.
Pentrite is also the substance that was found in the shoes of Richard Reid, a British man who allegedly tried to blow up an airliner heading from Paris to Miami with a bomb in his shoes.
Special anti-terrorism police are investigating the find, along with the counter-intelligence agency known as the DST and local police.
The weekly Royal Air Maroc flight, which arrived from Marrakesh late on Wednesday, returned to Morocco early on Thursday.
Marrakesh, in east-central Morocco, is one of the North African country's main tourist destinations.
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