Arriving on the red carpet of Masterpiece London. —Andy Barnham photo

LONDON – Gems reportedly worth $3.9 million were mysteriously swiped from the Masterpiece art fair during its last week’s run.

Even with high security, the crime went undetected at the stand of Swiss jeweler Boghossian. Staff discovered the gems were missing early on the morning of July 5.

Detectives are studying closed circuit television footage for clues. About 44,000 visitors attended the fair at the grounds of Royal Chelsea Hospital from June 29 to July 5.

A report in the Evening Standard said that the thieves apparently had a key to unlock secure cabinets, pointing to an inside job.

It said that detectives are hunting two men who calmly walked into the show and unlocked a cabinet before stealing several pieces of diamond jewelry.

The men, both white and casually dressed, then locked the cabinet at the stand before strolling away.