Israel’s Mossad spy agency planted explosives inside 5,000 pagers imported by Lebanese group Hezbollah months before Tuesday’s detonations, a senior Lebanese security source and another source told Reuters.

The Lebanese security source said the pagers were from Taiwan-based Gold Apollo, but the company said it did not manufacture the devices, but were made by a European firm with the right to use its brand.

Hezbollah has vowed to retaliate against Israel, whose military declined to comment on the blasts. The plot appears to have been many months in the making, several sources told Reuters.

The senior Lebanese security source said the group had ordered 5,000 beepers from Gold Apollo, which several sources say were brought into the country earlier this year.

Gold Apollo founder Hsu Ching-Kuang said the pagers used in the explosion were made by a company in Europe that had the right to use the Taipei-based firm’s brand, the name of which he could not immediately confirm.