Piper reported that as he was leaving Exchange Buildings to return to Houndsditch he saw a man acting suspiciously in the shadows of the cul-de-sac. As the policeman approached him, the man walked away; Piper later described him as being approximately 5 feet 7 inches (1.70 m), pale and fair-haired. When Piper reached Houndsditch he saw two policemen from the adjoining beats—constables Woodhams and Choate—who watched 120 Houndsditch and 11 Exchange Buildings while Piper went to the nearby Bishopsgate Police Station to report. By 11:30 seven uniformed and two plain clothes policemen had gathered in the locality, each armed with his wooden truncheon. Sergeant Bentley from Bishopsgate police station knocked at number 11, unaware that Piper had already done so, which alerted the gang. The door was answered by Gardstein, who made no response when Bentley asked if anyone was working there. Bentley asked him to fetch someone who spoke English; Gardstein left the door half-closed and disappeared inside. Bentley entered the hall with Sergeant Bryant and Constable Woodhams; as they could see the bottom of his trouser legs, they soon realised that someone was watching them from the stairs. The police asked the man if they could step into the back of the property, and he agreed. As Bentley moved forward, the back door opened and one of the gang exited, firing from a pistol as he did so; the man on the stairs also began firing. Bentley was shot in the shoulder and the neck—the second round severing his spine. Bryant was shot in the arm and chest and Woodhams was wounded in the leg, which broke his femur; both collapsed. Although they survived, neither Bryant or Woodhams fully recovered from their injuries.As the gang exited the property and made to escape up the cul-de-sac, other police intervened. Sergeant Charles Tucker from Bishopsgate police station was hit twice, once in the hip and once in the heart by Peters: he died instantly. Choate grabbed Gardstein and wrestled for his gun, but the Russian managed to shoot him in the leg. Other members of the gang ran to Gardstein's assistance, shooting Choate twelve times in the process, but Gardstein was also wounded; as the policeman collapsed, Gardstein was carried away by his accomplices, who included Peters. As these men, aided by an unknown woman, made their escape with Gardstein they were accosted by Isaac Levy, a passer-by, whom they threatened at pistol-point. He was the only witness to the escape who was able to provide firm details; other witnesses confirmed they saw a group of three men and a woman, and thought one of the men was drunk as he was being helped by his friends. The group, which included Peters, went to Svaars' and Peter the Painter's lodgings at 59 Grove Street (now Golding Street), off Commercial Road, where Gardstein was tended by two of the gang's associates, Luba Milstein (Svaars' mistress) and Sara Trassjonsky. As they left Gardstein on the bed, Peters left his Dreyse pistol under the mattress, either to make it seem the wounded man was the one who had killed Tucker, or to enable him to defend himself against a possible arrest.

Question: What is the name of the only witness to the escape?
Answer: Isaac Levy