Still, Margaret A. Burnham, a law professor and the founder of the Civil Rights and Restorative Justice Program at the Northeastern University School of Law, said these letters mattered, even if devoid of resolution.

“Setting aside whether the F.B.I. could have done more, I respect the dignity with which they are accepting responsibility for letting families know how justice failed them,” Ms. Burnham said. “Whether it’s a sufficient thing, I don’t know. But it’s a necessary thing.”