"Russia considered these people as very important to their intelligence-gathering activities," he told "Face the Nation" host Bob Schieffer in an interview broadcast Sunday.
He defended the decision to allow the 10 to return to Russia in exchange for the release of four Russian prisoners accused of spying for the West because the swap presented "an opportunity to get back ... four people in whom we have a great deal of interest."
White House press secretary Robert Gibbs, sidestepping the question of whether Russia's espionage poses a threat to the U.S., said the swap came amid improved relations between the two countries.
"The economic discussions that President (Dmitry) Medvedev and President Obama had just recently and the progress that we've made in reducing nuclear weapons - and hopefully we'll get a treaty through the Senate this summer that will further reduce nuclear weapons - means our security is stronger and safer and our relationship is stronger," Gibbs said on NBC's "Meet the Press."
The Attorney General confirmed to Schieffer the deported Russian spy network did not succeed in obtaining any U.S. state secrets.
"They didn't pass any classified information, and for that reason they were not charged with espionage," Hold said, adding, "the potential for what they might have done was, I think, a serious thing."
When asked by the Face the Nation host whether the group truly posed a security threat to the United States, Holder responded that, "as agents of a foreign power" the alleged spies "were certainly a threat".
The country's top lawyer also sought to erase concern over the fate of the children of the Russian agents, saying they all were allowed to return to Russia "consistent with their parents wishes" or, in the case of those who were adults or nearly adults, were allowed to make their own choices of where to live.
"The children have all been handled, I think, in an appropriate way," he said.
The seven offspring embroiled in the spy saga ranged in age from a 1-year-old to a 38-year-old architect. In most cases they were born and grew up in the United States, making them U.S. citizens.
More Spy Coverage:
CIA's Panetta, Russian Spy Chief Negotiated Swap
Spy Swap Discussed Weeks Before Arrests
Backgrounds of 4 Released by Russia in Spy Swap
U.S. Confirms Successful Exchange of Spies
Lawyer: Chapman Would Have Preferred to Stay
Judge Orders Spy Defendants Deported
Who Are Russians in Spy Case?
Defendants' Signed Plea Agreements
On pending terrorism cases, Holder acknowledged "there's a real question" as to whether a terrorist suspect such as self-professed Sept. 11 mastermind Khalid Sheikh Mohammed can face the death penalty if he were to plead guilty before a military commission.
Holder indicated he still favors bringing Mohammed and four alleged accomplices before civilian courts, but that has been. He said no decision has been made on where the trials will be held or whether they would be civilian or military.
He said one roadblock is that Congress has yet to come up with the money for the trials. "The politicization of this issue when we're dealing with ultimate national security issues is something that disturbs me a great deal," Holder said.
Holder also said the closing of the Guantanamo detention camp has become more difficult "because there have been people who have changed their positions" and Congress hasn't agreed to provide the money to relocate the detainees to an underused state prison in Illinois. He said other states have offered to take the prisoners, but did not name any states.
"There is no reason to believe that people held in Guantanamo cannot be held wherever we put them in the United States. Again, very safely and very effectively," Holder said.