Thousands of pagers have detonated across Lebanon, killing nine people and wounding nearly 3,000 others, including the group’s fighters and Iran’s envoy to Beirut, Reuters reports.

A senior Lebanese security source identified a photograph of the model of the pager, an AP924, which like other pagers wirelessly receives and displays text messages but cannot make telephone calls.

Hezbollah fighters have been using pagers as a low-tech means of communication in an attempt to evade Israeli location-tracking, two sources familiar with the group’s operations told Reuters this year. But the senior Lebanese source said the devices had been modified by Israel’s spy service “at the production level”.

“The Mossad injected a board inside of the device that has explosive material that receives a code. It’s very hard to detect it through any means. Even with any device or scanner,” the source said. The source said 3,000 of the pagers exploded when a coded message was sent to them, simultaneously activating the explosives.

Another security source told Reuters that up to three grams of explosives were hidden in the new pagers and had gone “undetected” by Hezbollah for months. Israeli officials did not immediately respond to Reuters requests for comment.