Bank robberies in Washington dropped abruptly in 1982, declining from 102 in 1981 to just 44 last year despite economic hard times that are often believed to cause such crimes.

Law enforcement officials say the reason for the drop is simple: the bank robbers are in jail--again.

Police say a small group of individuals operating independently of each other has been responsible for many of the bank robberies that have taken place in the city. During the early 1970s, they say, that group carried out a rash of bank robberies, with more than 100 such crimes occurring here each year between 1970 and 1974.

By 1975, police say, most of the persons responsible for those robberies were in prison, and the incidence of bank robberies dropped back into the middle double digits. Then in 1981, the numbers jumped again.

"What happened in 1981 was that a lot of these guys got out on parole and went back to their same routine," said Ron Dervish, a spokesman for the FBI. "A lot of them just needed a fast way of getting money. You find that most people who rob banks have heavy narcotics habits and are desperate to finance drugs that cost them $300 a day."

"Now we know who they are and they know D.C. is a tough place to rob a bank," said Sgt. Edward Dory, head of the D.C. police bank robbery unit.

Except for one man: the Bank Bomber.

"If we could just get him, our solution rate would jump," said Dervish. "We've done everything we could to come up with a lead but so far it's a dead end. Our best bet is that he is from the area and someone out there knows who he is."

The Bank Bomber struck at least seven times in 1982--more than any other individual--usually driving up to windows of Washington banks and threatening to blow up the tellers unless they gave him cash. There have been no injuries in any of the incidents.

So far, according to the FBI, the robber has netted $7,000, with most of that coming from the two times he actually entered a bank.

The suspect is described by law enforcement officials as a black man in his early 20s, between 165 and 185 pounds, who stands about six feet tall. Usually he drives up to a bank window and passes the teller a note saying he has a bomb. If a teller does not seem to believe him, he pulls out what police say appears to be a set of emergency flares wrapped with wire around an alarm clock. No one knows for sure whether the device could actually explode.

"The way it's rigged, we can't really tell what it is--but it's enough to make some tellers give him the money," said Dory. "Once, a teller just fell behind the counter to hide and the robber got nervous and drove away."

If it weren't for the Bank Bomber, FBI officials say, they would have closed more than 80 percent of the bank robbery cases in 1982.

"Right now he's the only one out there using that kind of MO [method of operation], and he's committed more bank robberies than any other person in the city for the year," said Dervish. "By using drive-in windows, he avoided getting his picture taken. But he also got very little money, so for his last jobs he changed up and went inside with a gun."

Dervish said the bank robber is believed to be from the Washington area, either the District or Northern Virginia.

While the suspect's use of a bomb may be unusual, he is in other respects typical of local bank robbers, police say. For example, most bank robbers here operate alone as compared with other cities where groups are more common.

Police say many local bank robbers have trouble lasting long at their trade because congested traffic downtown makes it hard for them to get away. But the risk to bank robbers does not end there.

A number of detection devices are now widely used by banks throughout the area that police say has made a dramatic difference in their ability to arrest suspects. Among them are improved cameras inside and outside some banks and a specially constructed money bag filled with dye and a small bomb that can be detonated by a time device or a laser beam.

"The exploding dye bag is great for leads in investigating bank robberies," Dervish said. "We have apprehended suspects with the dye still on them. If they put the bag inside their clothes and it explodes, they suffer serious injuries. Also, you can smell the stuff for blocks."