* Clerk confesses to stealing $6.5 mln over four years

* Stole to fund gambling addiction, says he worked alone

* World Cup betting losses proved the final straw

By Benet Koleka

TIRANA, Nov 8 (Reuters) - In the end, it wasn't the security cameras or the audit inspections in the vault of Albania's central bank that brought down Ardian Bitraj.

It was the high blood pressure and lack of sleep, the burden of a multi-million-dollar secret.

Sitting down with his boss this July, Bitraj confessed his deception: over a four-year period he had stolen the equivalent of $6.5 million from the vault, covering his tracks by stuffing the empty cash boxes with books and balls of string.

The revelation brought down the central bank governor, led to the arrest of 18 employees and tarnished the reputation of an institution once lauded for its professionalism. And all for the sake of a gambling habit that led to massive losses, culminating in a series of fatal bets on the soccer World Cup.

The full story of the Balkan bank heist is only just emerging, gleaned by Reuters in interviews with bankers, investigators and others involved, and from legal documents including a transcript of Bitraj's confession.

It started in May 2010, when Bitraj, who had risen to become head of the cash processing department at the bank, first opened the metal and plastic clasps to the wooden boxes that hold its cash reserves in the cryptically named X Building on the outskirts of the capital Tirana.

Bitraj, 45, had a penchant for placing bets on soccer matches, so roughly once a month he would wait for his co-workers to leave the room and swipe up to 2 million leks, roughly $18,000, according to the confession.

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Choosing carefully how he returned the boxes, Bitraj would make sure those he had tampered with were not in line for delivery to Albania's commercial banks, nor likely to be picked on in the regular random audit of the vault.

As the thefts mounted, he would stuff the boxes with packaging, balls of string and books to replace the weight of the cash.

All three keys needed to access the vault were kept in his personal safe. In statements to police, bank employees said they had not received any directive on how or where to store the keys.

Bitraj says auditors checked only 2 percent of the cash boxes in the vault. Fired governor Ardian Fullani says it was 5 percent, maintaining that checks in the former communist country were comparable with other central banks in Europe.

Witness testimony has since revealed that the bank's plumber and electrician both made up the numbers for the required seven-member audit team in 2012 and 2013.