Asked about his impressions of the claim that Zazi was in contact with an al Qaeda facilitator of the liquid explosives plot, Chertoff said, "What is concerning is the ability to recruit people who are legitimate U.S. citizens or residents and turn them. That has always been the real fear that we have faced over the past few years. We've begun to see that we've had some success in doing it although certainly not as much as they've had in Western Europe."
Zazi, who worked as an airport shuttle bus driver in Denver, was arrested after more than a year of surveillance by the FBI. He was caught with chemicals and hydrogen peroxide-based beauty products to make peroxide bombs, according to court documents filed in the case.
Zazi's father was indicted on Feb. 1 on charges that he helped conceal objects that Zazi was using in his bomb-making attempts.
During Friday's plea hearing, Ahmedzay, a New York City cab driver, admitted that he was involved in selecting the targets for subway bombings that allegedly were to have taken place within days after Sept. 11, 2009.
According to FBI officials, Zazi told agents interviewing him that he and his co-conspirators would try to bomb the 1, 2, 3 and 6 New York subway lines at rush hour.
"The facts disclosed today add chilling details to what we know was a deadly plot hatched by al Qaeda leaders overseas to kill scores of Americans in the New York City subway system in September 2009," Attorney General Eric Holder said in a statement released on Friday. During the hearing Ahmedzay said, "I'm thankful I did not do anything to harm anyone but I feel someone else might."
He also launched into anti-Semitic comments and made statements about the U.S. wars overseas in Iraq and Afghanistan.
Sentencing for Ahmedzay has been set for July, 30, 2010. Zazi is due to be sentenced in June.
ABC News' Luis Martinez contributed to this report.