Once inside the building, they disabled the elevator, leaving an “out of order” sign next to it, sent the elevator to the second floor, and shimmied down the elevator shaft to the basement, busting through a metal barrier. They cut a gray telephone cable jutting out of an alarm box, as well as the wires of an electrical box, disabling an iron gate protecting the vault, prosecutors say.

Then they began the long and arduous task of drilling through the vault’s wall, reinforced with concrete — a skill they had perfected by watching clips on YouTube. Shortly after 12:21 a.m. on April 3, Alok Bavishi, whose family owns the safety deposit company, received a call that the intruder alarm had been triggered. He testified that his concerns were initially tempered by the fact that a previous alarm had been triggered by an insect.

Kelvin Stockwell, a longtime security guard at the building, arrived nearly an hour later. After examining the front door and peering through the letter box of the fire escape door, he told the jury he decided that the building was secure and left without going inside.

The police were also notified of the alarm, but no response was deemed necessary. All the while, the thieves were in the basement, breaking into the vault. The police later apologized, saying the “call handling system and procedures for working with the alarm monitoring companies were not followed.”

Even so, the gang’s luck proved short-lived. When they finally breached the wall against which the metal cabinet holding the safe deposit boxes was standing, they were stopped in their tracks because the cabinet was bolted to the ceiling and floor, and they were unable to dislodge it.

They eventually left around 8 a.m., empty-handed. But they were undeterred, returning two days later, on Mr. Perkins’s 67th birthday. The burglary may have gone unnoticed because the security deposit’s neighboring businesses were closed for the Easter weekend. But the ease with which the thieves left and then returned, undetected, has led some to speculate that the crime was an inside job.