(NewsNation) — The accused architect of the Sept. 11 terror attacks may never face the death penalty.

The United States Department of Defense and FBI have told victims’ families that plea agreements are under consideration for Khalid Sheikh Mohammed and four other defendants.

That doesn’t mean they’ll ever get out of prison. The five men have been in custody for nearly two decades.

A notice was made in a letter obtained by NewsNation that was sent to several of the 9/11 families. The Pentagon and the FBI advised that a plea deal remains a possibility. The letter was dated Aug. 1, though some families received the correspondence just last week. They’re being asked to respond by Monday with any questions.

The letter comes more than a year after military prosecutors and defense lawyers began exploring a negotiated resolution.

The prosecution of those held at the U.S. detention center in Guantanamo Bay, Cuba, has been troubled by repeated delays and legal disputes. No trial date has been set.

“Why doesn’t this country, why doesn’t this government stand up for the 9/11 community and put us first?” said Terry Strada, chair of 9/11 Families United. “I don’t know how much longer, how much more pain and suffering they think the 9/11 community can take.”

Kristen Breitweiser, whose husband died in the south tower, said the lack of full prosecution is inconceivable.

“My husband was murdered, 3,000 people were murdered in broad daylight, and to think that this government, my country is not going to fully prosecute anyone for that, I just think it’s a failure of this country,” she said.

The Associated Press contributed to this report.