Holder said in his opening remarks that he wanted to clear up some misconceptions that have arisen since he announced Friday that Mohammed and four others detained at Guantanamo Bay, Cuba, would be tried at a lower Manhattan federal courthouse, where they would face a possible death penalty.
He sought to assure lawmakers that classified material would be protected during the trial, the defendants would be held securely, and Mohammed would have no more of a platform to express his beliefs than he would have had at a military trial.
"I'm not scared of what Khalid Sheikh Mohammed has to say at trial and no one else needs to be afraid either," Holder said. "We need not cower in the face of this enemy."
But committee ranking member Jeff Sessions, R-Ala., said, "I don't think the American people are overreacting," and cited concerns raised by current and former officials, including former New York City Mayor Rudy Giuliani, who called the decision "dangerous" and "irresponsible" in a Fox News interview on Monday.
Holder has said his decisions between trials and commissions were based strictly on which venues he thought would bring the strongest prosecution.
The transfer of the suspects from Guantanamo to New York is still weeks away, and is part of President Obama's pledge to close the detention center in Cuba. About 215 detainees are still there, according to the Associated Press.
The administration has already sent one detainee, Ahmed Ghailani, to New York to face trial.
Ten other high-profile Guantanamo detainees are being transferred to the United States for trials.
In addition to Mohammed, who admitted to masterminding the 9/11 terrorist attacks, four other detainees are heading to civilian court in New York:
Waleed bin Attash, a Yemeni known as Khallad, allegedly ran an al-Qaida training camp in Logar, Afghanistan, where two of the 19 hijackers were trained, according to the AP.
Ramzi Binalshibh, a Yemeni, allegedly helped find flight schools for the hijackers, helped them enter the United States and assisted with financing the operation.
Ali Abd al-Aziz Ali, also known as Ammar Al-Baluchi, who was born in Pakistan and raised in Kuwait, allegedly helped nine of the hijackers travel to the United States and sent them $120,000 for expenses and flight training.
Mustafa Ahmad al-Hawsawi, a Saudi, allegedly helped the hijackers with money, Western clothing, traveler's checks and credit cards. He testified in the trial of Zacarias Moussaoui, saying he had seen Moussaoui at an al-Qaida guesthouse in Kandahar, Afghanistan, in early 2001, but was never introduced to him or conducted operations with him, the AP reported.
Five others are headed for military commissions in the United States on a variety of terrorism charges.
Toronto-born Omar Khadr was 15 years old when he was captured after allegedly killing an American soldier during a 2002 battle in Afghanistan.
Ahmed Mohammed al-Darbi, who allegedly has met with Osama bin Laden, trained at an al-Qaida terrorist camp and plotted to blow up a ship in the Strait of Hormuz or off Yemen.
Ibrahim Ahmed Mahmoud al Qosi is accused of acting as al-Qaida's accountant, paymaster and supply chief during the 1990s when the terror network was centered in Sudan and Afghanistan.
Noor Uthman Muhammed is allegedly a member of al-Qaida, and trained at a camp in Afghanistan and later become a weapons instructor.
Abd al-Rahim al-Nashiri, a Saudi of Yemeni descent, is the alleged mastermind of the 2000 bombing of the USS Cole, reported the AP.
---- Compiled from wire reports and other media sources