New York (CNN) -- Two suspects in an alleged plot to bomb New York's subway system pleaded not guilty Thursday to additional charges in the case, as prosecutors announced there would potentially be more arrests in the investigation.
Jeffrey Knox, an assistant U.S. attorney, gave more details about the alleged plot in federal court Thursday. He told the court the suspects planned "three coordinated bomb attacks on the Manhattan subway during rush hour last September."
"It was to be similar to the al Qaeda London attacks in July of 2005," Knox said.
Those attacks killed 52 people.
He also said more charges are likely in the case and that "additional defendants are likely from overseas."
Zarein Ahmedzay and Adis Medunjanin pleaded not guilty to new charges of conspiracy to use weapons of mass destruction against persons or property in the United States, as well as several other counts.
The two had previously faced lesser charges.
Prosecutors say the men -- 25-year-old U.S. citizens and residents of the Queens borough of New York -- conspired with Najibullah Zazi "to attack the New York subway system in mid-September 2009."
FBI agents searched Ahmedzay's home Thursday morning for bomb-making materials and other evidence, according to a law enforcement official with knowledge of the case.
Zazi pleaded guilty this week to conspiring with others to blow up New York subway targets with homemade bombs.
A federal grand jury issued a new indictment charging Ahmedzay and Medunjanin on charges of conspiracy to commit murder in a foreign country, providing material support to a foreign terrorist organization -- al Qaeda -- and receiving military-type training from al Qaeda, prosecutors said in a statement Thursday.
Knox told the court the defendants "received military training under control of al Qaeda leadership."
Ahmedzay also faces a charge of making false statements to the FBI, the statement said.
Two of the four charges that both men face -- conspiracy to use weapons of mass destruction and conspiracy to commit murder in a foreign country -- carry a maximum sentence of life in prison. They face up to 15 years in prison if convicted of providing material support to a foreign terrorist organization. The maximum penalty is 10 years in prison for receiving military-type training from al Qaeda.
The two men were indicted last month on the lesser charges and pleaded not guilty. The charges that federal prosecutors detailed Thursday come from a fresh indictment that supersedes, or replaces, the original charges.
Ahmedzay worked as a taxi driver while Medunjanin, a Bosnian-American, was employed as a building manager. Both were friends with Zazi, a native of Afghanistan who lived in Colorado.
Zazi pleaded guilty this week to conspiring to use weapons of mass destruction, conspiracy to commit murder in a foreign country and providing material support to a terrorist organization
"In spring 2008, I conspired with others to join the Taliban, to fight along with the Taliban against the United States," he said in court Monday. "We were recruited to al Qaeda instead."
The threat of legal action against Zazi's associates and family played a role in his decision to cooperate with the government, a law-enforcement source told CNN. The source said Zazi's parents faced potential immigration fraud charges. They were not facing deportation but did face the possibility of criminal penalties.
Zazi's father, Mohammed Wali Zazi, initially was charged with lying to investigators, but in January a federal grand jury in New York charged him with conspiracy to obstruct justice by helping to discard bomb-making chemicals when he learned of the government's investigation.
A judge is scheduled to sentence Zazi June 25. He faces a maximum sentence of life in prison for the first two counts and an additional 15 years in prison for the third count, the Justice Department said in a statement.
"This attempted attack on our homeland was real," U.S. Attorney General Eric Holder said at a news conference Monday, praising the criminal justice system in foiling the plot. "It was in motion, and it would have been deadly."