A 38-year-old Washington man suspected in almost 50 downtown holdups since September has been arrested by detectives who cornered him in an airlines office after employes there activated an electronic spotting device during a robbery attempt.

Detectives with guns drawn rushed into the United Airlines office at 1725 K St. NW Tuesday afternoon, shouted "April Fool!" and slapped handcuffs on Henry Johnson, listed at 1515 45th St. NE. He was charged with robbery.

"You got me," police said Johnson muttered as he stood in the office clutching an airline ticket envelope that employes had just stuffed with hundreds of dollars.

With the arrest of Johnson, a nattily dressed, tough-talking suspect dubbed the "Candy Man" because of his alleged penchant for robbing candy stores, police say they have now marked closed at least 47 downtown robberies staged since September.

In all cases, police said, the holdup man's manner, appearance and method were similiar -- a sharp dresser in camel's hair coat, black knit cap, velour shirt and custom-made slacks entering the place of business, barking terse orders and keeping his hand thrust in his pocket as though he had a pistol.

Johnson's arrest Tuesday marked the culmination of months of sleuthing and thousands of man hours of stakeouts by 10 two-man police teams using special battery-powered spotting devices planted in selected downtown business establishments where they hoped to intercept their quarry.

The device -- called Tact II -- was activated by a United employe pressing a hidden pedal during the robbery attempt at the K street office Tuesday. It lit up an electronic panel in a command car operated by a police lieutenant cruising the area.

By radio, he instructed detectives staked out in offices adjacent to the United Airlines office that a robbery was in progress.

The detectives rushed in and Johnson immediately surrendered, police said. He was not armed.

Police say Johnson is suspected of robbing the same place March 21.

"As soon as the guy walked in the door, I knew him," United employe Sara Godowns said yesterday. "He sadi, 'I'm back for the rest of the money, and this time I want it all.'"

Godowns said Johnson then ordered another employe, Nancy Weilmunster, to give him the cash. During the robbery, a customer came in, asked for routine information and left, apparently unaware of the near-silent drama.

Johnson then ordered the employes into a backroom where one of them activated the Trac II device, Godowns said. Police arrived moments later.

Police said Johnson is wanted for parole violation in connection with a prior robbery conviction and is being held in jail pending notification of parole officials.