GUANTANAMO BAY NAVAL BASE, Cuba -- A prosecutor asked a judge Tuesday to allow public testimony at Guantanamo Bay this fall from some people who had family members killed in the Sept. 11 terrorist attack, saying otherwise they may never get the chance.

The military tribunal for the five Guantanamo Bay prisoners charged in the case has been grinding along in the pretrial stage since May 2012. But the trial itself could still be years away.

So prosecutors want to bring 10 people to the U.S. base in Cuba in October to testify in open court about the loss of their sons and daughters on Sept. 11, 2001.

The 10 are considered important "victim impact" witnesses by the prosecution. But because of their age or health, they may not be around for a trial by military commission that could be years away. "This is of critical importance to us," federal prosecutor Ed Ryan told the judge in a hearing at the base.

Defence lawyers say public depositions from the families would taint the potential military jury pool and should be done in private.

All five defendants face charges that include nearly 3,000 counts of murder in violation of the law of war and terrorism for their alleged roles planning the attack and providing logistical support to the hijackers. They are being tried by military commission, a hybrid of the military and civilian court system, and could get the death penalty if convicted.

Prosecutors say the trial could start in 2017 at the earliest and defence lawyers say it will likely be several years later because of all pretrial legal issues that remain to be resolved.

The prosecution request to depose the witnesses in open court came on the second day of a weeklong hearing at the base and the judge, Army Col. James Pohl, was not expected to issue an immediate ruling on the issue.

Witnesses that the prosecutors would like to call include Lee Hanson, 83, whose son was on the hijacked United Airlines flight that was hijacked and crashed into the South Tower of the World Trade Center and died along with his wife and 2-year-old daughter, the youngest victim of the terrorist attack. Another is John Vigiano, 77, who lost two sons, one a New York City firefighter, the other a police officer. Two people who the prosecution had wanted to call as witnesses have already died, Ryan told the court. They are among about 400 victim family members who have volunteered to testify in the case.

The testimony from the family members is "undoubtedly important evidence for the commission members to consider when evaluating the harm caused by the actions of the accused," the prosecutors argued in a court filing.

Defence lawyers say the testimony may not even be admissible in the guilt phase of the trial, and only in a potential penalty phase. Introducing it earlier would make it even harder to defend men are already facing "inherent" prejudice of being accused of the worst terrorist attack in U.S. history.

"Why would you make the process of providing a level playing field even marginally more difficult? There's not a reason to do that," said David Nevin, the lead civilian lawyer for Khalid Shaikh Mohammad, the self-proclaimed mastermind of the plot.

Another concern of the defence is politics. Attorney James Harrington, who represents defendant Ramzi Binalshibh, noted that the October testimony would take place just before the U.S. presidential election. "It seems to me that the timing of this motion is just incredible," he said. "It is right before a national presidential election and if this is public it is really going to be public."

Ryan dismissed concerns raised by the defence that such public depositions would taint the potential pool of jurors, who will eventually be selected from the officer ranks of the U.S. military, all of whom would likely already know all about what happened on Sept. 11 or may have been personally affected by it in one way or another.

"It was one of the most infamous crimes in history and by their design it happened in such a way that the whole world was watching," he said.