Raees Alam Qazi (22) and Sheheryar Alam Qazi (32) face a potential statutory maximum sentence of 35 years and 20 years respectively, they said yesterday.
The brothers, naturalised US citizens from Pakistan, also confessed assault of two federal officers while in custody in 2014.
Southern District of Florida Judge Beth Bloom has set June 5 date for sentencing of the duo in the terror plot.
According to the Justice Department, the Qazi brothers acknowledged that Raees, the younger brother, was going to initiate an attack using a weapon of mass destruction in New York City and that he had been financially and emotionally supported by his older brother, Sheheryar, who encouraged him to launch the attack.
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In January 2015, a federal grand jury added additional terrorism charges and five counts of conspiracy, assault and attempted murder relating to an attack on two Deputy US Marshals in April 2014 while the Qazis were in federal custody.
"With today's guilty pleas, Raees Qazi and his brother Sheheryar Qazi are being held accountable for their roles in a plot to conduct a terrorist attack using a weapon of mass destruction in New York City and their assault on two federal officers during their pretrial detention," said Assistant Attorney General John Carlin.
"The Qazi brothers later attacked federal law enforcement agents.
"As today's guilty pleas demonstrate, we will respond by holding those who plan terrorist acts on American soil accountable.
This case serves as an example of our commitment to protecting civilians from violent jihadi attacks," he said.