Israeli security forces launched a manhunt Monday for six Palestinian prisoners who escaped overnight from one of the country’s highest-security prisons through a hole in their bathroom.

A security official said the fugitives fled down a shaft beneath the bathroom floor and then sneaked through underground passages in the prison’s foundation.

Israeli police responded in large numbers, erecting roadblocks after the rare jailbreak from the Gilboa prison in northern Israel. Security forces patrolled streets in the north of the country and the occupied West Bank, as helicopters flew above.

Prime Minister Naftali Bennett described the escape as a “grave incident.” Arik Yaacov, northern commander of the Israeli Prisons Service, said an investigation was underway and the six men appeared to have found a flaw in the facility rather than tunneled the entire way out. It was not immediately clear whether they had help from outside to orchestrate the escape.

The Gilboa prison, located about two miles from the West Bank, is one of the most heavily guarded in the country.

The men on the run range in age from 26 to 49 years old, the Palestinian Prisoners Society said. At least four had been serving life sentences, with one of them detained since 1996.

The Israeli Prisons Service said five of the prisoners had links to the Islamic Jihad movement, which called the breakout “a heroic act that will shock the Zionist defense system.” The group commended them in a statement for “snatching their freedom with their fingertips from under the eyes and ears of the occupier.”

Among the six fugitives is Zakaria Zubeidi, 46, who had been jailed since 2019 and was a former commander of the al-Aqsa Martyrs Brigades in the West Bank town of Jenin during the second Palestinian uprising, which began in September 2000.

Authorities were planning to move 400 inmates to other prisons to avoid other jailbreak attempts, Israel’s Army Radio said Monday.

Photos circulated in Israeli media of a hole in the ground outside the prison, purporting to show the end of the escape route.