The Israeli spy agency Mossad has placed a small number of explosives inside 5 thousand Taiwanese-made locators ordered by the Lebanese group Hezbollah months before Tuesday’s blasts, a senior Lebanese security source and another source told Reuters.

Details shed light on an unprecedented security breach by Hezbollah that saw thousands of radars detonated across Lebanon. nine dead and nearly 3,000 injuredincluding fighters from the group and the Iranian ambassador to Beirut.

Hezbollah has vowed to strike back at Israel, whose military has declined to comment on the bombings.

The plot appeared to have been in the works for months, several sources told Reuters.

Lebanon’s top security source said the group ordered 5,000 pagers made by Taiwan’s Gold Apollo, which several sources said were brought into the country in the spring.

A Lebanese security source has found a photograph of the AP924 locator model, which, like other devices of this type, receives and displays text messages over the wireless network but cannot make phone calls.

Hezbollah militants used pager as a low-tech media outlet trying to circumvent Israeli localization, two sources familiar with the group’s activities told Reuters this year.

However, a Lebanese source said the devices had been modified by the Israeli spy service “at the production level.”

“Mossad inserted a plate with explosive material into the device, which received a code. It is very difficult to detect it by any means. Even with any device or scanner,” the source said, adding that 3,000 locators exploded when they were sent a coded message, simultaneously activating the explosives.

Another security source told Reuters that the new pagers contained up to three grams of explosives and that Hezbollah had not found them for months.

Neither Israel nor Gold Apollo immediately responded to Reuters requests for comment.

Images of the destroyed pagers analyzed by Reuters showed formatting and stickers on the back that matched the devices made by Taipei-based Gold Apollo. (Reuters)