Israeli authorities were conducting a widespread manhunt on Monday for six convicted terrorists from Gilboa prison who reportedly used their cell’s drainage system to tunnel their way to freedom.

Defense Minister Benny Gantz’s office said reinforcements were sent to border crossings and “to the area around the frontier, while preparing to conduct whatever actions are needed to catch the terrorists, in close coordination with the Shin Bet security service and other security services,” according to the Times of Israel.

Gilboa prison, which is about two miles from the occupied West Bank, is considered one of the country’s most secure and its prisoners consist of those convicted or accused of deadly attacks, Reuters reported. Prime Minister Naftali Bennett’s office called the escape a “grave incident that requires an across-the-board effort by the security forces.”

The army said the prisoners included Zakariye Zubeidi, a former militant leader in the northern West Bank city of Jenin, as well as three Islamic Jihad militants serving life sentences for involvement in deadly attacks on Israelis.

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Zubeidi was a leader in the Al Aqsa Martyrs Brigade, affiliated with the Fatah movement, during the second Palestinian uprising over 20 years ago.

The Associated Press contributed to this report