By James Copnall
BBC News, Rabat
The probe at Kenitra will ask whether there were accomplices
The escape of nine Islamist prisoners convicted of terrorist offences is an embarrassment for Morocco.
The Moroccan government presents stopping the Islamist menace within as one of its key challenges.
It is keen on convincing countries like the US, Spain and France it is an ally in the war against terrorism, and the Moroccan security services have been mobilised to hunt for the fugitives.
Several of the bombers who struck in Madrid in 2004 were from Morocco, and disaffected young men leave the country to fight in Iraq in relatively large numbers.
In February, the Moroccan police said they had broken up a large terrorist cell that planned to attack politicians and prominent Moroccan Jews.
This is the first time sentenced terrorists have managed to escape from a Moroccan jail.
The men who escaped had been convicted of belonging to terrorist groups that carried out the 2003 Casablanca bombings. The attacks killed 45 people, including 12 suicide bombers.
The Moroccan authorities are not saying how the men escaped from Kenitra prison, north of Rabat.
But an expert on Islamic groups, Moroccan professor Mohamed Darif, told the BBC the men used a tunnel to get out.
"It is astonishing how they got away," he said.
"They used a tunnel which came out at the house of the director of the prison.
"They must have had accomplices."
It was not immediately possible to get confirmation of Prof Darif's account but an interior ministry source told the AFP news agency the prisoners had used a tunnel.
Before they fled, the prisoners apparently wrote a heartfelt message proclaiming their innocence on the wall of the prison toilets.
Abderahim Mouhtad heads an association that supports Islamic prisoners
He says other prisoners still in Kenitra told him the written message insisted the nine had done no wrong.
The escapees wrote that they had tried every legal means to prove their innocence, and, as this had failed, they were left with no choice but to flee.
Human rights organisations and many Moroccans believe that many of the Islamists convicted after the Casablanca bombings were innocent.
The escaped prisoners' message, Mr Mouhtad said, ended by stating that they had received no help from other prisoners or from prison guards during their breakout.
Eight prison guards had been convicted of helping drug dealer Mohammad Ouazzani escape from the same jail last December.
There is certain to be an investigation into how the "Kenitra Nine" were able to escape.
"They could be a threat," warned Prof Darif.
"We are now confronted with people who are extremely disappointed, they believe they are innocent and have been mistreated. They could do anything."
The breakout coincided with a hunger strike by many of the 900 or so Islamist prisoners currently in Moroccan jails.
They are protesting about the poor conditions they say they suffer, conditions frequently criticised by foreign and local human rights organisations.