The tip came in: 31 mysterious blue plastic barrels, a gas-siphoning pump, a wrench used to open and seal large drums, and six red plastic containers normally used to store gasoline had been spotted being delivered to a single-family brick house in Upper Northwest.

Fearing someone was amassing explosive materials, federal agents filed an affidavit and got a judge to sign a search warrant. And at dawn yesterday, the agents, assisted by D.C. police explosives experts and an emergency response team, descended on the 3000 block of Birch Street NW.

But instead of bombs, they found the family had stockpiled empty barrels--in preparation for Y2K.

"You just never know these days," said Special Agent Jeffrey Roehm, a spokesman for the Bureau of Alcohol, Tobacco and Firearms, which led the raid.

In a four-page affidavit filed in U.S. District Court asking for the search warrant, an ATF special agent noted that today is the one-year anniversary of the bombing of U.S. embassies in Kenya and Tanzania.

And, the affidavit continued, the number of barrels seen being delivered to the house "is consistent with the construction of 'car bombs' of the type similar to bombings that have occurred both in the United States and in the Middle Eastern area of the world."

Agents feared the barrels were being used to store a potent brew of ammonium nitrate and fuel oil. An Iowa company run by two men reputed to be "survivalists" had also made a delivery to the house, alarming investigators.

"We decided to be cautious and to make sure there was no danger to the public," Roehm said. Referring to the April 1995 blast that killed scores of people inside a federal office building, he added, "If you flash back on Oklahoma City, that's what they used: large blue plastic drums."

A woman who answered the phone at the home last night said the 7:30 a.m. raid "was frightening. . . . I wish they had done more research . . . before coming in like this."

The woman, who said she was visiting her three sons, ages 25, 23 and 21, who live at the house, said the brothers decided to store the containers in case of Y2K problems.

She said her 23-year-old son, a computer consultant, conceived the idea. "You know how it is--computer people are more aware that things are going to go down," she said.

Calling the family "very cooperative" Roehm said agents concluded that the containers were for water.

"We check these things out to make sure there's no problem," he said. "And when there isn't, we all walk away a lot happier."