The following article contains an answer for the question: Which person was working with an undercover police officer to open the safe? , can you please find it? 

John Arthur Lennox is an underworld chieftain who runs a nightclub where society patrons come to rub elbows with the criminal set. He is nicknamed Silk because of his fondness for the expression that things are "fine as silk". From the club he directs a violent $50,000 bank heist and cheekily invites in two detectives to establish his alibi. While the robbery is going on, he gets his new singer Jimmy Lambert to play a recording over the intercom that makes it appear as if his men are on the premises.
When Deacon, the gangster holding the stolen money, tries to skip town, Silk has him killed at the train station. The money is not found on his body, but the gang suspects that it is hidden in the express office. Meanwhile, Jimmy has realised that the recording will help convict Silk of the robbery and with Nola, the dance partner in his act, plans to use it against him.
As the law closes in and his allies turn against him, Silk is arrested but has to be released when witnesses, afraid of reprisals, refuse to identify him. In the police line-up, Silk meets Fingers Smalley, who agrees to break open the express office safe. After establishing an alibi at the club, Silk and Fingers leave for the office, but the police arrive as Fingers opens the safe. Silk is killed during the ensuing gunfight and Fingers explains that he is really an undercover police agent named Ferguson.

John Arthur Lennox