The Stolen Painting

Detective Parker was called to investigate the theft of a valuable painting from an art gallery. The painting, worth millions, was taken during the gallery's annual exhibition. The detective interviewed the staff and gathered the following information:
Security footage showed three people entering the gallery after hours: Mr. White, the janitor; Ms. Green, the curator; and Mr. Black, the security guard.
Mr. White claimed he was cleaning the restrooms during the time the painting went missing.
Ms. Green said she was in her office organizing paperwork during that time.
Mr. Black stated he was doing his rounds and checking the security cameras.
Can you help Detective Parker solve the case and determine who stole the painting?

Solution: After reviewing the statements and evidence, Detective Parker realized that Mr. Black, the security guard, was lying. Since Mr. Black claimed to be checking the security cameras during the theft, he should have noticed the painting being taken on the footage. Since there was no sign of the thief on the security footage, it indicates that Mr. Black was involved in the theft. Therefore, Mr. Black is the thief who stole the painting.