Robert Lill, 65, a retired FBI agent who was involved with numerous high-profile cases in the Washington area in the 1970s, died Sept. 12 from a severe neck fracture he received while surfing with a friend at Ocean City.

Mr. Lill's wife and the friend's wife were watching from the beach during the accident, which published reports said might have been the result of rip currents caused by recent tropical storms and hurricanes in the Atlantic Ocean.

Mr. Lill, who lived in Silver Spring and Ocean City, was co-lead investigator in a joint FBI/D.C. police investigation of the "Freeway Phantom Murders" in 1971 and 1972, a string of sexual assaults and murders that claimed at least seven young black girls; their bodies were dumped near or along freeways in the District and Prince George's County.

Despite a massive investigation into what might have been the first serial killings in the District of Columbia, the cases never were solved.

Mr. Lill also participated in the Watergate investigation and served as a co-lead investigator of an FBI/D.C. police investigation into the 1973 shooting and robbery of Sen. John C. Stennis (D-Miss.). The veteran lawmaker was shot in the chest and left leg in front of his Washington home; two men were later apprehended.

Perhaps Mr. Lill's most successful operation during his years in the Washington field office as an undercover investigator was a series of stings in 1976 and 1977.

The FBI and local law enforcement agents worked the stings four times to locate and arrest fences trafficking in stolen goods and peddling narcotics.

In the first operation, which the press dubbed "the Sting," undercover officers wrapped up a five-month investigation by luring criminals to a phony Saturday-night party in a Northeast Washington warehouse -- with the promise of meeting a Mafia "don" who had come down from New York to purchase their stolen goods.

Once there, the partygoers were advised to approach the don as he sat in a high-backed chair and kiss his ring. "Bless you, my son," intoned the don, actually a Washington police sergeant, shortly before the suspects were collared.

About 160 people were arrested, including two D.C. jail guards, a federal prosecutor, a postal worker and a gang of thieves who stole cars out of showrooms. They brought in goods worth $2.4 million; police bought the goods for $67,000.

"It succeeded beyond our fondest hopes," Mr. Lill told The Washington Post at the time.

He helped arrange four operations in all, including GYA ("Got Ya Again") and Highroller, which operated out of the Shoreham American Hotel and targeted high-level professional thieves with connections to organized crime.

Another sting used a fully stocked ice cream truck that sold ice cream to children while also dealing with thieves.

"Bob had a flair and a talent for chasing the things that were very difficult to track down," Edward R. Leary, a former FBI colleague, said in a phone interview.

"He had a skill for bringing agencies together that normally didn't work together," Leary added. "He was the linchpin."

After his tenure as a special agent and supervisory special agent in the FBI's Washington field office, Mr. Lill became a supervisor at FBI headquarters. In that capacity, he was the architect of undercover policy and procedure.

Before his retirement in 1988, he lectured frequently on undercover operations to law enforcement groups across the country. In retirement, he occasionally served as a backgrounds investigator for the FBI and also worked for the Laborers' International Union of North America.

Robert Elmer Lill was born in Rochester, N.Y. He grew up intending to be a priest and received his undergraduate degree in 1960 from St. Bernard's Seminary in Rochester. He received a master's degree in history from the University of Detroit in 1963. He was a lifelong jazz enthusiast.

He joined the FBI in 1966 and served the bureau in many capacities. When he retired, he was chief of the undercover and special operations unit of the criminal division at FBI headquarters.

He was active in volunteer programs at St. John the Baptist Catholic and St. Michael's Catholic churches in Silver Spring. He also volunteered at St. Matthew's Cathedral in Washington to assist the homeless.

Survivors include his wife of 41 years, Margaret S. Lill of Silver Spring and Ocean City; four children, Mary Euerle of Herndon and Robert M. Lill, Dr. John T. Lill and Gregory M. Lill, all of Silver Spring; and nine grandchildren.

FBI agent Robert Lill participated in Watergate and "Freeway Phantom Murders" investigations.