By Stephen Coonts

Pocket Books. 408 pp. $19.95

What's under siege in this, Stephen Coonts's fourth and most ambitious thriller, is Washington, D.C., site of the upcoming trial of Aldana, the recently nabbed and extradited kingpin of Colombia's drug cartel. Aldana (estimated net worth: $4 billion) puts his captor nation on notice that he does not intend to go quietly: "I am your worst nightmare come to life," he says menacingly during a cellblock press conference.

And in Coonts's hands, Aldana more than makes good on his promise. An airliner is blown up. A skilled assassin begins to methodically work down his hit list of American officialdom, starting with the president. Terrorist squads armed with Uzis turn a Metro station and the corridors of the Capitol into abattoirs. The city's power substations are destroyed. Martial law is declared. Troops are deployed. Tanks roll. Washington is under siege.

Coonts spins out this nightmare in compelling, graphic fashion ("The killer stood calmly amidst the blood and gore and groaning victims and changed magazines. He emptied the second magazine into the prostrate crowd and was inserting the third one into his weapon when a guard appeared in the doorway ..."), and layers the narrative with some interesting subplots, ranging from the efforts of an undercover cop working his high-wire act into the confidences of a local crack kingpin to an uprising in Castro's Cuba. (Attempts to introduce tangential, topical issues such as the S&L crisis, the laundering of drug profits and those monsters of the Beltway -- political action committees -- are less successful, though the oily lawyer/PAC-man and the senators who are the beneficiaries of this narco-largess are colorful if not fully convincing).

Where "Under Siege" falls down completely is in the use of local landmarks. A closed savings and loan is sited in Alexandria -- go figure -- "over in Maryland." The assassin's New Hampshire Avenue hideaway appears on occasion on "Hampshire Avenue." The Washington Monument rises above the "Reston skyline." (Either the libel lawyers said, "Obscure everything," or the dog ate the street map.)

You certainly wouldn't want to get directions from these guys. But you wouldn't want to end up in their cross hairs, either. Witness the Beltway rub-out of one Harrington: "Tony swung the rifle gently, adjusting for the jolts of the car. His finger tightened on the trigger. Harrington's head exploded as the rifle bellowed."

Coonts's depiction of a Washington unraveled -- Beltway entrances blocked, boulevards empty, troops going house to house in search of the terrorists -- is chilling: "By two p.m. every traffic circle in the District had a tank parked in the flower beds beside the statue. The olive-drab monsters sat in pairs upon the Mall, the diesel engines idling in the chill December wind as the crews stood nearby drinking coffee from disposable cups and looking with wide eyes at the sprawling buildings bathed in the weak winter sun."

Aside from having crafted a first-rate narco-political thriller, Coonts, through Navy Capt. Jake Grafton (a Coonts mainstay in a reduced role in this book, bordering on ennui) raises some sobering questions about the glib real-world assumption that the military is superbly suited to serve as a domestic police force as well as some of the Draconian proposals for interdicting drugs, such as shooting down suspicious private aircraft: "Cessnas and Pipers going down in flames over the Florida beaches will make great television," Grafton observes dryly. "The doctors and dentists had better find someplace else to fly," retorts the chairman of the Joint Chiefs.

"Under Siege" is a disquieting story, told by a first-rate storyteller who, so far, has resisted grinding out formulaic bestsellers. Coonts has a keen eye for detail -- especially in his descriptions of the methodical preparations by the assassin, the grisly execution of a local drug kingpin's lieutenant, and Grafton's attempted track-down of the assassin (though the showdown is a little "Mod Squady"). The only sour note is the name of the president the fictional assassin is stalking: It's George Bush. That hits too close to home.

The reviewer, a Washington attorney, is a frequent contributor to Book World.