D.C. police Detective James Trainum leans over a rusty guardrail and stares into a den of weeds, trees and brush that snarls an embankment of Interstate 295 in Southeast Washington.

As cars flash past, Trainum mulls over question after question. Why did the killer toss his first two victims down this hill? Was he pressed for time? Did this spot, across from Bolling Air Force Base and near St. Elizabeths Hospital, mean something to him?

For two years, Trainum has sifted through old police and FBI reports, read faded newspaper clippings, hovered over embankments and interviewed victims' relatives. He is trying to do what has eluded three generations of investigators: crack the most notorious unsolved serial killing cases in District history.

During a 16-month period that began in spring 1971, the Freeway Phantom, as he came to be known, killed six females, ages 10 to 18, three with the middle name Denise. At least three were raped, and every one of them was strangled. Their bodies were found on or near busy roads or highways in the District or Maryland.

A cold case detective who is a fan of intellectual challenges, Trainum has a job made more complicated because most of the police files are incomplete and all of the physical evidence has been lost or destroyed, ruling out today's sophisticated forensic tools.

"I know this is a long shot," Trainum says. "But we live for long shots."

Thirty-five years ago, the Freeway Phantom slayings triggered one of the largest investigations the region has seen. Two dozen detectives were assigned to the hunt initially, and the FBI was called in -- until Watergate diverted the agency's manpower. The failure to solve the homicides continues to haunt families of the victims. And current and former investigators find they can't dislodge the Phantom from their minds.

In reexamining the slayings, The Washington Post reviewed thousands of FBI files and police reports and interviewed dozens of former and current detectives, witnesses and victims' family members. A note recently obtained by The Post and never before published shows how bold and taunting the killer became after abducting his fifth victim. The note, tightly guarded by investigators for many years, was found in the dead teenager's coat pocket.

"This is tantamount to my insensititivity [sic] to people especially women," reads the note, which police determined was dictated by the Phantom and written by his victim. "I will admit the others when you catch me if you can! Free-way Phantom."

The slayings of the victims -- all black and seemingly chosen at random -- came during a time of political and racial tumult in the city. Washington did not yet have home rule. And it was still reeling from the 1968 riots that followed the assassination of Martin Luther King Jr. More than 70 percent of the District's 757,000 residents were black, and there was widespread distrust of the police department, which was more than 60 percent white.

Although the racial makeup of the police department has changed -- now, the chief and many top commanders are black -- anger lingers among the victims' relatives.

"You better bet that if these had been white girls, the police would have solved the cases," says Evander Spinks, a sister of the Phantom's first victim. "They didn't care about us. All the cases involving white girls still get publicity. But ours have been forgotten."