Another complaint against Stephen Ambrose has emerged.

This one dates back to 1970, when fellow historian Cornelius Ryan accused him of a "rather graceless falsification" in Ambrose's book, The Supreme Commander. Ambrose acknowledged failing to give Ryan proper credit for some material, but the mistake remains in the book's current edition.

The allegations were first reported Tuesday on Forbes.com.

At least five other books by Ambrose have been questioned. He has blamed the errors on his recent prolific pace — Ambrose has averaged more than a book a year since the mid-1990s — but the latest incident involves one of his earlier works.

The dispute centers on two quotations, from two different men, that first appeared in 1966 in Ryan's The Last Battle. Ambrose used those passages the following year, and credited them properly, in Eisenhower and Berlin, 1945. But in The Supreme Commander, published by Doubleday in 1970, Ambrose ran the quotes together, as if they were spoken by the same person, and failed to cite Ryan's book as the source.

Writing in 1970 to then-Doubleday editor-in-chief Kenneth McCormack, Ryan complained of Ambrose's "rather graceless falsification."

"The crowning indignity, as far as I am concerned, is that in his notes on sources in the back on page 710 he gives credit to — guess who? None other than his own previous book, Eisenhower and Berlin, 1945, page 21," wrote Ryan, author of such best sellers as The Longest Day. He died in 1974.

Just as Ambrose did recently when a professor at the University of Pennsylvania complained Ambrose's The Wild Blue had lifted material, the historian apologized to Ryan.

"It was a careless error," he wrote. "I should not have run the ... quotes together, and I should not have cited my own work as a source. I offer no defense, and only hope that similar errors do not appear elsewhere."

Ambrose promised the errors would be corrected, but the most recent edition of The Supreme Commander, published in 1999 by the University Press of Mississippi, still credits the quotes to Ambrose's book. The mistake of attributing them to the same person was fixed.

A spokesman for Ambrose said the historian would have no immediate comment. A call to the University Press was not immediately returned Tuesday