“We have always known that locating the subjects, witnesses and evidence for 40-year-old murder cases was going to be challenging, and that many of these matters would not be prosecutable,” said Xochitl Hinojosa, a spokeswoman for the Justice Department.

But families yearn for action. Over the years, relatives of Louis Allen, a civil rights worker who was ambushed at his front gate in Liberty, Miss., in 1964, have met with countless officials and offered a $20,000 reward.

“Everybody put me on the back burner for years and years,” said Mr. Allen’s son, Henry C. Allen, 65. He had hoped the cold case initiative would change things, but said the response has been slow. He said the legwork had been done. “Here’s the people you can contact, here’s their phone number, here’s their address,” he said. “I don’t have the authority to go knock on their door, but you do  and it still don’t get done.”

Special Agent Cynthia Deitle, chief of the F.B.I.’s civil rights division, said that agents were not permitted to share the progress of continuing investigations with families or the public. Witnesses, she said, might not always be forthcoming about having cooperated with the F.B.I. The Allen case remains open.

The Till bill authorized up to $13.5 million a year to solve racially motivated murders before 1970, but in fiscal year 2009 no money was allocated. In fiscal year 2010, the Justice Department received $1.6 million for civil rights cold cases. The F.B.I. received an $8 million increase for its civil rights division, which handles human trafficking, hate crimes, present-day civil rights violations and cold cases, a spokesman said.

Officials have repeatedly insisted that lack of money was not an obstacle. “There’s never been a resource issue whether it was people or money on our end,” Agent Deitle said.