BEIRUT, Oct 16 (Reuters) - The batteries inside the weaponised pagers that arrived in Lebanon at the start of the year, part of an Israeli plot to decimate Hezbollah, had powerfully deceptive features and an Achilles' heel.

The agents who built the pagers designed a battery that concealed a small but potent charge of plastic explosive and a novel detonator that was invisible to X-ray, according to a Lebanese source with first-hand knowledge of the pagers, and teardown photos of the battery pack seen by Reuters.

To overcome the weakness - the absence of a plausible backstory for the bulky new product - they created fake online stores, pages and posts that could deceive Hezbollah due diligence, a Reuters review of web archives shows.

The stealthy design of the pager bomb and the battery’s carefully constructed cover story, both described here for the first time, shed light on the execution of a years-long operation which has struck unprecedented blows against Israel's Iran-backed Lebanese foe and pushed the Middle East closer to a regional war.

A thin, square sheet with six grams of white pentaerythritol tetranitrate (PETN) plastic explosive was squeezed between two rectangular battery cells, according to the Lebanese source and photos.

At some point, Hezbollah noticed the battery was draining faster than expected, the Lebanese source said. However, the issue did not appear to raise major security concerns - the group was still handing its members the pagers hours before the attack.

On Sept. 17, thousands of pagers simultaneously exploded in the southern suburbs of Beirut and other Hezbollah strongholds, in most cases after the devices beeped, indicating an incoming message.

Among the victims rushed to hospital, many had eye injuries, missing fingers or gaping holes in their abdomens, Reuters witnesses saw, indicating their proximity to the devices at the time of detonation. In total, the pager attack, and a second on the following day that activated weaponised walkie-talkies, killed 39 people and wounded more than 3,400.

Two Western security sources said Israeli intelligence agency Mossad spearheaded the pager and walkie-talkie attacks.

Reuters could not establish where the devices were manufactured. The office of Israel's Prime Minister Benjamin Netanyahu, which has authority over Mossad, did not respond to a request for comment.

Lebanon’s Information Ministry and a spokesperson for Hezbollah declined to comment for this article.

Israel has neither denied nor confirmed a role. The day after the attacks Israeli Defence Minister Yoav Gallant praised Mossad's "very impressive" results in comments that were widely interpreted in Israel as a tacit acknowledgement of the agency's participation.