He has been variously called "Basil" or "the ghost," a mystery man with red hair and a lanky frame who appears to have played a crucialrole in the largest burglary in England's history - before disappearing.

Over Easter weekend last year, when four aging thieves stole $20 million in gold, jewelry and gems at the Hatton Garden Safe Deposit bank in central London, a video camera recorded the man, about six feet tall and wearing a cap. The police suspect that he opened a fire-escape door that allowed the rest of the gang to enter the building, and that he knew the door codes and sought to disable the alarm protecting the safe deposit boxes.

Now, the mystery of Basil's identity has taken a twist after one of the four ringleaders of the theft, Daniel Jones, 60, wrote a letter to Sky News saying that Basil was a former police officer who was involved in private security.

"Basil was the brains, as I was recruited by him," Mr. Jones wrote in the letter, published this week, from Belmarsh prison in southeast London, where he is awaiting sentencing after pleading guilty. "He let me in on the night of the burglary, he hid keys and codes throughout the building."

The burglary at Hatton Garden, led by the four white-haired thieves who were hoping to fatten their pensions, has captivated Britons and drawn headlines around the world.

In April, Mr. Jones; John "Kenny" Collins, 75; Terry Perkins, 67; and the gang's elder statesman, Brian Reader, 76, committed what they hoped would be one last career-topping heist. After sliding down an elevator shaft at 88-90 Hatton Garden in London's diamond district, they used power drills to bore a hole through a reinforced concrete wall. Then they ransacked 73 of more than 900 safe deposit boxes, taking jewelry, gold, cash and precious stones.

The heist has attracted particular notice because of the ages of the men, who proved to be ill equipped for the digital age. Mr. Reader took the bus to the burglary. The operation was meticulously plotted with the help of the book "Forensics for Dummies." But their apparent ignorance of modern-day police techniques contributed to their undoing. The men continued to use their cellphones, even after the theft, helping the police trace them.

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