A Seven Minute Mystery: The Movie and the Robbery

Here is another Seven Minute Mystery. These are quick mystery scenarios designed to exercise your cranium juices. No Google. No Internet search. No Wikipedia. Just you and your cranium. Give yourself only seven minutes to read each one and solve it. The answer will be in a future post.

What are you waiting for? The clock’s ticking . . .

It took only five minutes on a hot August night for Johnson to kill Andre Cleo and rob him of $3,000.  Seven minutes before eleven p.m., the time the main feature at the corner movie theater began, Johnson stepped across the hall and knocked at Cleo’s door.  Being admitted, Johnson closed the door behind him.  Cleo stared questioningly at Johnson’s black-gloved hands; his eyes opened wide as Johnson drew his gun.  The silencer whistled twice.  Cleo fell to the floor.

Taking the dead man’s keys, Johnson opened Cleo’s strongbox and put $3,000 in cash into his pocket.  Then he dialed the police.  “My name is Johnson,” he said.  “I’m calling from Mr. Cleo’s apartment at 591 Grand Street.  There’s been a murder!” 

Hanging up, Johnson slipped out of Cleo’s apartment, dropped the gun down the garbage chute, and went across the hall to his own room.  There he removed his gloves and hid the money. 

 He was waiting in Cleo’s room when Inspector Neidermeyer arrived.  Johnson said:

“It was too hot to sleep, so I decided to go to a movie.  As I stepped into the hall a big man dashed out of Cleo’s door, knocked me down, tossed something into the garbage chute, and raced downstairs.”

 That night the Inspector related the case to Dr. Wells, concluding with: “Johnson claimed he had not been in Cleo’s apartment for a week prior to tonight when he called us.  But after our lab boys got through there, Johnson confessed.  I suppose you can guess why.”

QUESTION: Explain what evidence the lab produced and why it caused Johnson to confess.