The authorities also determined the devices, which included pagers and hand-held radios, were detonated by sending electronic messages to the devices, says the letter, seen by Reuters on Thursday. Israel was responsible for the planning and execution of the attacks, Lebanon's U.N. mission said.
The 15-member Security Council is due to meet on Friday over the blasts.
The attacks on Hezbollah's communications equipment on Tuesday and Wednesday killed 37 people and wounded around 3,000, overwhelming Lebanese hospitals and wreaking bloody havoc on the group.
Israel has not directly commented on the attacks, which security sources say were probably carried out by its Mossad spy agency, which has a long history of carrying out sophisticated attacks on foreign soil.
