According to Reuters, citing informed sources, Israel’s Mossad spy agency planted explosives in 5,000 pagers imported by Hezbollah before the devices were detonated on Tuesday.

The sources told Reuters the plot had been in the works for several months, with the pagers brought into Lebanon earlier this year.

A senior Lebanese source quoted by Reuters said Israel’s spy service had modified the devices “at the production level.”

“Mossad inserted a board inside the device containing explosive material that receives a code. It’s nearly impossible to detect with any device or scanner,” the source explained.

They also noted that 3,000 pagers exploded after receiving a coded message, triggering the explosives simultaneously.

Another security source told Reuters that up to three grams of explosives were hidden in each pager, going "undetected" by Hezbollah for months.