The Case of the Missing Diamond Ring

Detective Anderson was tasked with solving the case of the missing diamond ring from a jewelry store. The ring was kept in a glass display case with a digital lock, accessible only by the store owner and the salesperson. The surveillance footage showed the following:
At 10:00 AM, Mr. Thompson, the store owner, opened the store and unlocked the display case to showcase the jewelry.
At 10:15 AM, Mrs. Johnson, a customer, entered the store and browsed through the jewelry.
At 10:30 AM, Miss Martinez, the salesperson, attended to Mrs. Johnson and helped her try on various rings.
At 10:45 AM, Mr. Thompson briefly left the store to attend to an urgent matter.
At 11:00 AM, Mrs. Johnson left the store after deciding not to make a purchase.
At 11:15 AM, Miss Martinez noticed the diamond ring was missing from the display case.
Who stole the diamond ring?

Solution: Mrs. Johnson stole the diamond ring. She had the opportunity to take it while trying on various rings with the assistance of the salesperson, Miss Martinez.