Jury selection is scheduled to begin Sept. 29 in federal court in Camden.
Lee, a reserve in the Army Criminal Investigation Command, is scheduled to deploy to Iraq on Sept. 14 and return Jan. 5. His mission is to search for soldiers missing in action.
The defendants, all foreign-born Muslims raised in the Philadelphia area, have been accused of plotting an attack inspired by al-Qaeda.
Two FBI informants recorded hundreds of hours of conversations with the men, including discussions of attacking Fort Dix and other possible targets.
Defense attorneys have signaled their plan to assail the credibility of the informants and possibly argue that they entrapped the defendants.
The five men, Mohamad Shnewer, Serdar Tatar and brothers Shain, Eljvir and Dritan Duka, face life in prison if convicted of plotting to kill soldiers.
In addition to taking the stand, Lee would have watched the informants testify and helped defense attorneys with cross-examinations.
Lee learned of his deployment in July, and defense attorneys tried to hire two other experts to replace him, without success.
They said they were still looking for another expert, but that few with Lee's credentials were willing to take on the case.
"Therefore, obtaining a replacement . . . is an extremely difficult - and to date impossible - task," the attorneys wrote.
Lee has spent 140 hours preparing for the case, and even if another expert could be found, the witness would need "significant time to get up to speed," the attorneys said.
Defense attorneys asked that the trial be delayed, but did not suggest a date.
The next status conference in the case is scheduled for Sept. 11. U.S. District Judge Robert Kugler could rule then, if not sooner.
Kugler has been pushing for a speedy trial since the suspects were arrested in May 2007. He initially hoped to start the trial in 2007.
A spokesman for the U.S. Attorney's Office said he had no comment on the defense request.
Among Lee's credentials, he served as the agent-in-charge of the DEA's office in Karachi, Pakistan, from 1996 to 1998. Before that, he was an embassy attache in Islamabad, Pakistan.
Lee also participated in the arrest of 1993 World Trade Center bombing suspect Ramzi Yousef, and testified at his trial. Yousef was convicted.
Lee retired from the DEA in 2003 and became an author and a consultant in California.
Previously, Kugler suggested that Lee could testify via satellite from Iraq. The defense yesterday said that option was "rife with difficulties," and that Lee was "extremely apprehensive" about providing testimony in the case while on active military duty.
The defense said that "clearly anyone can understand that it would also make Mr. Lee a 'persona non grata' among his fellow military personnel, even if such a reaction would be unwarranted."
Contact staff writer Troy Graham at 856-779-3893 or firstname.lastname@example.org.