A bank robber struck out yesterday morning at the Riggs National Bank in Georgetown. But the man -- who police say may be responsible for several other recent bank holdups -- wasn't going to wait out the long holiday weekend before striking again.

The man robbed the Riggs bank about 9:30 a.m. But after a dye pack in the money bag exploded, the robber abandoned his loot outside the bank, in the 1200 block of Wisconsin Avenue NW, and hustled across town to a Citibank in Brookland, D.C. police said.

Wearing the same white shirt and blue denim shorts, the man walked into the Citibank branch in the 3800 block of 12th Street NE about an hour later, police said. As he had done at Riggs, the man handed a holdup note to a teller and brandished a gun. The teller handed over an undetermined amount of money. The man fled, possibly in a brown or bronze-colored car, police said.

Now D.C. police and the FBI are trying to determine whether they have another serial bank robbery operation on their hands. As many as seven recent robberies in the District, including the two yesterday, may be the work of the same person or persons, police said.

This year, 19 bank robberies have been reported in the District, said Capt. Michael E. Reese, commander of the D.C. police unit that investigates bank robberies. Six have been linked to the group of men charged last month in a conspiracy to rob banks by using high-powered weapons and carefully planned escape routes.

Investigators were at the scene of the Georgetown robbery yesterday when the report of another holdup on the other side of town came over the radio. It didn't take long for police and the FBI to link the two robberies.

"We put the puzzle together," Reese said.

Investigators marveled at the robber's boldness in striking twice in such a short span of time. "I would say it's pretty brazen," Reese said.

Dye packs, such as the one that tripped up the robber in yesterday's first holdup, are typically placed among the bills handed over by a teller during a bank robbery. When the pack explodes, it can mark the money and the robber.

Yesterday, an employee at a nearby Georgetown restaurant was in the alley behind the bank as the robber fled and the dye pack went to work.

"It just started exploding," said the employee, who spoke on the condition of anonymity because he is a witness. "He just dropped the bag and kept running."

The employee wasn't sure at first what was in the bag, which was engulfed in a cloud of dust. But the employee, concerned because the bag had been dropped near another employee's vehicle, tried to get a little closer.

"The stuff just burned my eyes," the witness said. "I was like, 'Damn, what was that?' Then I saw it was a money bag and I said, 'Damn, they just robbed somebody,' and I called 911."

Police hope to release a surveillance photograph of the robber in the next few days. They ask anyone with information about the robberies to call 202-727-9099.