The alleged architect of the September 11, 2001 attack and several other defendants could avoid the death penalty under plea agreements being considered, according to a letter to families from the Pentagon and FBI.

The letter was sent by federal agencies to several families of the thousands of people killed during the 9/11 attacks. It comes a year and a half after military prosecutors and defense lawyers began looking at a potential resolution to the case.

Khalid Sheikh Mohammed as well as four other people are being held at Guantánamo Bay, Cuba. The case has been hindered and delayed, particularly over the legal issues of the interrogation under torture that the men went through while in CIA custody.

"The Office of the Chief Prosecutor has been negotiating and is considering entering into pre-trial agreements," states the letter. 

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Khalid Shaikh Mohammed, the alleged Sept. 11 mastermind, is seen shortly after his capture during a raid in Pakistan Saturday March 1, 2003 in this photo obtained by the Associated Press.

This Saturday March 1, 2003, photo obtained by The Associated Press shows Khalid Shaikh Mohammad, the alleged Sept. 11 mastermind, shortly after his capture during a raid in Pakistan. (AP Photo) (AP)

It continues to say that a plea agreement hasn't been finalized "…and may never be finalized, it is possible that a PTA in this case would remove the possibility of the death penalty."

The letter, which was received by some family members, is dated Aug. 1 and asks them to respond by Monday to the FBI's victim services with their thoughts on a plea agreement.

Mohammed allegedly presented the idea of an attack on the United States to al-Qaida leader Osama bin Laden, according to the 9/11 Commission. The four other defendants allegedly supported the hijackers in multiple ways.

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Tribute in Light 9/11 New York City

The Brooklyn Bridge 9/11 Tribute in Light in New York City (Fox News Photo/Joshua Comins)

2,977 people died as a result of the 9/11 attack when two planes hit the World Trade Center and another one hit the Pentagon. A fourth plane was heading toward Washington, D.C. but crashed in Pennsylvania.

Jim Riches, who lost his son who was a firefighter during 9/11, said that he laughed bitterly upon opening the letter.

"How can you have any faith in it?" Riches said, adding that the update "gives us little hope."

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September 11 memorial new York city

NEW YORK, NY - SEPTEMBER 11:  New York City Police Officer Danny Shea, a military vet, salutes at the North pool of the 9/11 Memorial during the tenth anniversary ceremonies of the September 11, 2001 terrorist attacks at the World Trade Center site, September 11, 2011 in New York City. New York City and the nation are commemorating the tenth anniversary of the terrorist attacks on lower Manhattan which resulted in the deaths of 2,753 people after two hijacked planes crashed into the World Trade Center.  (Photo by David Handschuh-Pool/Getty Images) (David Handschuh-Pool/Getty Images)

"No matter how many letters they send, until I see it, I won’t believe it," said Riches. 

The Associated Press contributed to this report.