WASHINGTON  In a highly unusual admission of error, the Justice Department acknowledged on Wednesday that government lawyers should have known that Congress had recently made the rape of a child a capital offense in the military and should have informed the Supreme Court of that fact while the justices were considering whether death was a constitutional punishment for the crime.

“It’s true that the parties to the case missed it, but it’s our responsibility,” the department’s public affairs office said in a statement.

“We regret,” the statement said, “that the department didn’t catch the 2006 law when the case of Kennedy v. Louisiana was briefed.”

In that case, decided June 25 by a vote of 5 to 4, the court ruled that the Constitution prohibits the death penalty for the rape of a child. Justice Anthony M. Kennedy’s majority opinion was based in part on the conclusion that because child rape was a capital offense in only six states, and not under federal law, the death penalty for the crime did not meet the “evolving standards of decency” by which the court judges capital punishment.