The batteries of the walkie-talkies used by Lebanese armed group Hezbollah that blew up this week were laced with a highly explosive compound known as PETN, a Lebanese source familiar with the device’s components told Reuters.

The way the explosive material was integrated into the battery pack made it extremely difficult to detect, the source said.

Hundreds of walkie-talkies used by the group exploded on Wednesday, a day after thousands of Hezbollah’s pagers detonated across the group’s strongholds in Lebanon.

The Lebanese source said explosions had occurred even in cases where the battery pack was separated from the rest of the device.