Shortly before 3 a.m. on Feb. 17, 1984, an improvised bomb, made of five metal pipes packed with explosives that were wired to a 9-volt battery connected to a Sunbeam kitchen timer, exploded at a Norfolk abortion clinic, causing $1,000 damage.

During the next 11 months, seven abortion clinics and related facilities in the Washington area were hit by bomb blasts, the most recent a Southeast Washington clinic in the first hours of 1985.

The rash of attacks triggered an investigation that peaked in a frantic few days of work just before Jan. 22 of this year, the 12th anniversary of the Supreme Court's controversial decision legalizing abortion, when investigators feared that the bombers would strike again.

Three Maryland men were arrested Jan. 19 after an investigation in which agents for the federal Bureau of Alcohol, Tobacco and Firearms became convinced that the attacks were related when they found similarities in the construction and components of the bombs. Tracing the sale of materials by various companies in the Baltimore-Washington area, agents learned the identities of two of the suspects, while an anonymous tip from a jogger led them to the third.

The men -- Thomas Eugene Spinks, 37, Michael Donald Bray, 32, both of Bowie, and Kenneth William Shields, 34, of Laurel -- have been charged with conspiring to bomb the clinics. Shields and Bray are free on bond. Spinks remains in jail in Baltimore.

Federal prosecutor Justin Williams said during a bail hearing for Shields last week in federal court in Alexandria that the alleged conspiracy also involves firebombings at a clinic in Dover, Del., and at the Prince George's Reproductive Service in College Park. Both attacks took place early last year.

Bray has denied the charges, and the two other men and their families have refused to comment. In a jailhouse interview the day after his arrest, Bray claimed that he had been "framed" by ATF agents and said he was committed to opposing abortion through nonviolent methods.

While many details of the probe that led to the arrests remain secret, at least a partial picture can be pieced together from court documents, testimony and interviews with law enforcement and other sources.

"It was basic dogged digging, a lot of fruitless-lead type police work," an ATF official involved in the investigation said last week.

First, ATF chemists and bomb experts discovered a similarity in the construction and components of the bombs, as detailed in an affidavit filed by ATF in the case. Many of the bombs contained a homemade explosive mixture of sugar and sodium chlorate that came from a home welding kit. Five used fire extinguisher cylinders as casings, and at least two were attached to Sunbeam kitchen timers.

But the major breakthrough in the case did not come until after a Nov. 19 blast at the Metro Women's Medical Center in Wheaton.

The bombing provided two leads, one of them a fluke.

Two days prior to the bombing, abortion protesters staged a sit-in at the clinic and were featured in local television newscasts. On the morning of the bombing, a jogger anonymously told one of the television stations that as he ran through the clinic parking lot minutes before the blast, he saw a person there who resembled one of the protesters he had seen on the newscast, according to testimony of ATF agent John R. Schworm at Bray's bond hearing last week. The jogger said the person resembled the only protester holding a child, Schworm said, and agents reviewed the news film and determined that the protester was Bray.

Agents placed Bray, a house painter and copastor of the Reformation Lutheran Church in Bowie, under surveillance. More than a month later, on Jan. 1, a bomb exploded at the Hillcrest Women's Surgi-Center in Southeast Washington, where agents found three strands of hair on a piece of tape attached to the bomb device. Schworm testified that agents asked Bray for a hair sample and found that it matched the hairs at the bomb scene in "over 25 characteristics."

The second lead came when agents found a serial number -- 876 -- on a fragment of a fire extinguisher cylinder used as the bomb casing in the same Nov. 19 blast.

According to an affidavit filed in federal court, agents said they traced the number to a Hyattsville company, F&M Fire Protection Service Inc., which reported selling the cylinder, along with 33 others, to an individual identifying himself as "Lou Burns."

F&M employes described "Burns" as a "white male, 5 feet 8 to 5 feet 9 inches, slender build with thinning brown hair," who drove a battered green sedan, the affidavit states.

At that point, agents did not know who "Burns" was, according to an ATF official familiar with the investigation.

But the name had surfaced in interviews with chemical company employes throughout the area where agents had asked about large purchases of the materials used to concoct the explosives.

On July 12, 1984, for example, a person identifying himself as "Lou Burns," a representative of "Shield's Industries," picked up an order of 300 pounds of sodium nitrate, one of the ingredients found in the bomb residue, from a Baltimore chemical company, Maryland Chemicals, the affidavit states.

Another chemical company, Inland-Leidy Inc. of Baltimore, reported dealings with "Shields Industries," the affidavit states. On July 10, 1984, a person identifying himself as "Ken Shields" picked up 50 pounds of sulfur commercial flour from Inland-Leidy.

Two days later, a person identifying himself as "Ken Shields" of "Shields's Industries" ordered 400 more pounds of chemicals that were never picked up. Sulfur residues were found at four of the five sites that were bombed after July 10, according to an ATF affidavit.

"Lo and behold, there was a Shields Industries" in Laurel, an ATF official said in an interview last week. The firm, listed in the phone book, is believed to be a metals company run by Kenneth William Shields.

Agents located Shields, who had just started a new job at Virginia Wire & Fabric in Warrenton. He was living with his wife and infant daughter at a Holiday Inn there.

In a 1 1/2-hour interview Jan. 18 in the restaurant of the Holiday Inn, Shields told ATF agent John J. Mellon that he was the Ken Shields who purchased the sulfur flour from Inland-Leidy on July 10, Mellon testified at a bond hearing for Shields last week.

Shields said he had been accompanied by Thomas E. Spinks, and that the two men were planning to start a mail-order chemical business, Mellon said.

Having been given Spinks' name, agents then matched his fingerprints on a firearms permit with a print found on a piece of black tape at the scene of a Nov. 3 bombing at the American Civil Liberties Union office on Capitol Hill, according to the affidavit and ATF sources.

In addition, according to the affidavit, an ATF document examiner found that it was "highly probable" that Spinks had printed the name "Lou Burns" on one of the chemical company receipts.

On the morning of Jan. 18, ATF agents following Bray saw him buy $15 worth of gas at an Exxon station in Seabrook, Md., according to testimony by ATF agent Schworm at Bray's bail hearing. Forty-five minutes later, Bray drove to another station on Greenbelt Road in Greenbelt, next door to the Metropolitan Family Planning Institute, which had been the target of bomb threats and where Bray had picketed, Schworm said.

Bray bought $1.50 worth of gas there, but he remained at the station for 15 minutes, according to Schworm. Federal prosecutor Robert J. Mathias said in court last week that it could be inferred that "Bray had the clinic under surveillance."

Bray's lawyer, Robert Muse, argued at the hearing that Bray's actions that morning did not indicate any intention to bomb the clinic. He said that Bray, "as a frequent protester," often visited clinics prior to demonstrations in order to plan the way picketing would be conducted.

As the pace of the investigation quickened on Jan. 19, three days before the anniversary of the Supreme Court abortion ruling, agents following Spinks saw him buy pyrotechnic fuses at a hobby shop in Waldorf, the affidavit states. Spinks, who is identified in the affidavit as 5 feet 8 inches and 140 pounds, drove a 1973 green Mercury sedan. Remnants of pyrotechnic fuses had been found at several of the bomb sites, according to the affidavit.

And Shields, in a second interview in Warrenton Jan. 19, conceded knowing about many of the attacks in advance, Mellon testified. He said Shields admitted giving Spinks "technical information" about "chemical formulations," as well as several manuals describing how to make explosives. But Shields' lawyer has argued that the information and manuals were easily accessible and that buying the sulfur flour was legal.

The three men were arrested later that day.