French anti-terrorism investigators are questioning three Islamic radical convicts amid suspicions they helped another prisoner plot an attack on prison guards that has prompted national concern.

The attack Sunday in the prison of Osny north of Paris left two guards hurt, and has sparked criticism of special prison units that group together Islamic radicals. The attacker used a razor to slash one guard's throat and wounded another guard in the arm before being subdued with a rubber bullet.

A terrorism inquiry was opened, and the Paris prosecutor's office said the attacker and three others were in special custody Wednesday as authorities try to determine whether it was part of a broader plot.

France, targeted on multiple occasions by Islamic State extremists, has long had problems with radicalism in prisons.