Two guards at Ketziot Prison in southern Israel were wounded, one seriously, when Hamas prisoners stabbed them late Sunday night after clashes broke out at the facility.

Both guards received first aid at the scene, after which the seriously wounded guard was evacuated to Soroka Medical Center in Beersheva with a neck wound. The other guard was stabbed in his hand.

After the guards were attacked, riots reportedly broke out, during which a number of prisoners were injured. Five prisoners were evacuated to Soroka for treatment.

Sunday’s attack marked the most serious incident in a week of unrest at the nation’s prisons. Last month, security prisoners set 10 cells at the Ramon Prison on fire.

The Israel Prison Service reported that searches earlier this year of prison wings where security prisoners are held turned up some 70 contraband cellphones.

The IPS sees cellphones as a tool to use in remote direction of terrorist activity, as well as a way for terrorist leaders to communicate.

Some of the phones were found hidden inside prisoners’ body cavities. These prisoners, who were arrested for relatively minor security offenses, swallowed the phones prior to their detention with the intention of using them once they passed out of their bodies.

The prison service said it was currently working to install cellphone signal-jammers in security wings of prison facilities, which is causing tension among the prisoners.