Manhattan ? Historian David McCullough says it’s time to “drop the subject” on accusations of plagiarism against his friend and fellow historian Steven Ambrose.

McCullough, author of the biographies “John Adams” and “Truman,” said Friday at Kansas State University that he was baffled by the allegations against Ambrose that had emerged.

Ambrose, who has been accused of plagiarism in six of his works, admitted at a lecture Thursday in St. Louis that he mistakenly used only footnotes and not quotation marks around other writers’ work used in his books.

McCullough, a Pulitzer Prize winner, defended Ambrose during an appearance at the Landon Lecture Series.

“I don’t know why this has happened, and I don’t think Steven knows either,” McCullough said. “He has made a lot of great contributions to his country, and I think it’s time to drop the subject.”

McCullough made no mention of his own research controversy. Harper’s Magazine recently criticized McCullough for writing in “John Adams” that Thomas Jefferson had called the second president a “colossus of independence,” something Jefferson never said.

McCullough later admitted he had erred, telling The Associated Press in an interview, “It’s hard work; you’re trying to get the truth about distant times. When you make the mistakes, it’s very painful, but you will make mistakes. We’re imperfect, in an imperfect world.”

McCullough also praised rescuers and others involved in the aftermath of the Sept. 11 terrorist attacks, telling an audience that the courage of the nation’s forefathers was present in America today.

“We saw the utmost value and incredible courage of the unsung: the firemen, the policemen and the ironworkers. Those young men on the flight over Pennsylvania showed us the right stuff that this young generation is made of.”

McCullough is currently working on a book about the year 1776, which he said he became interested in during his research on Adams.