Common threads of physical evidence found at bombing sites, including sugar and chemical traces from home welding kits and metal fragments of carbon dioxide cylinders, led federal agents to arrest two of the three men charged Saturday in connection with the bombings of eight abortion-related facilities in the District, Maryland and Virginia, according to sworn statements filed in federal court.

A 15-page affidavit filed Saturday in U.S. District Court in Hyattsville by a special agent for the federal Bureau of Alcohol, Tobacco and Firearms details similarities in bits of explosive devices culled from the debris of abortion clinics and other blast sites, including an American Civil Liberties Union office in Southeast Washington.

The document also reveals how ATF agents used handwriting samples from receipts for purchases of large quantities of chemicals and interviews with sales clerks to build their case.

ATF agents arrested Thomas E. Spinks, 37, and Michael Donald Bray, 32, both of Bowie, and Kenneth William Shields, 34, of Laurel, Saturday on charges of conspiracy to violate federal explosives and firearms laws and "making and using improvised explosive devices" to bomb seven abortion clinics and related facilities and the ACLU office. Bray is not mentioned in the affidavit, which was signed by ATF Special Agent John R. Schworm.

According to that affidavit, which was filed in support of a request for a search warrant for Spinks' house, Shields told investigators last week that he and Spinks purchased large quantities of chemicals and that Spinks had taught him to make bombs out of used carbon dioxide cylinders and explosive compounds.

The affidavit stated that the explosive devices used in a number of the bombings were constructed of empty carbon dioxide cylinders packed with an explosive material that contained, among other things, sodium chlorate derived from home welding kits and sugar.

The affidavit also states that a lone fingerprint on the back of a piece of tape found at the site of the ACLU blast led federal agents to begin an investigation of Spinks. In addition, the affidavit says that in the opinion of an ATF document expert, "it is highly probable" that a name on a receipt for a purchase of a quantity of chemicals last September was printed by Spinks.