No joke -- funnyman David Letterman's office was grim yesterday as staffers wondered which of their colleagues played cat burglar over the weekend, stealing $4,000 from a locked desk drawer.

Nervous employees were preparing to be grilled by police, who say Sunday's brazen burglary looks like an inside job. 

"We're treating this seriously," said a show source. "People are pretty shaken up about it." 

Not only did the as-yet unidentified perpetrator know where the show kept its petty cash, but he also knew the location of a discreet security camera that could have revealed his identity, sources say. 

"They're going to interview people who work there, but there's no suspect at this time," police spokesman Sgt. Kevin Hayes said yesterday. 

According to sources, a lone burglar wearing gloves and a one-piece coverall with a hood entered the side door of the Ed Sullivan Theater building at 1697 Broadway sometime Sunday. 

He walked directly to a hidden security camera and covered the lens with tape and paper. 

The segment caught on security video was so brief that police cannot even determine his race, police sources said. 

The man went to an office where petty cash is kept in a production aide's locked desk drawer. 

It was unclear if the desk was located near Letterman's office, but one source said the pilfered desk was not that of Letterman's personal assistant. 

The amount of cash on hand last weekend was not considered unusual, a source said. 

Such cash is used for last-minute purchases and expenses, including some of the show's elaborate sight gags, which are sometimes assembled on one day's notice. 

Letterman, who often alludes to events in his private life in his Late Show monologues and conversations, didn't joke about the burglary last night.

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