With terrorism fears heightened following the discovery of another plot to blow up a plane using an underwear bomb, the American Civil Liberties Union is trying to have one of the government's tools against terrorism ruled unconstitutional.

The no-fly list has been around since 2003 and has grown to roughly 20,000 names, including about 500 U.S. citizens. It's maintained by the FBI's Terrorist Screening Center, which also keeps a terrorist watch list with more than 500,000 names.

The ACLU filed suit in 2010 and will argue this week the case should be heard in U.S. District Court in Portland. “In the modern day, air travel is absolutely fundamentally important to how we travel on a daily basis,” says ACLU attorney Nusrat Choudhury, “and that right to travel is protected by the Constitution's promise of due process.”

The case involves 16 plaintiffs who are all U.S. citizens and Muslims. They argue the government refuses to explain why they were put on the list. It's a point the TSC does not dispute. An official told Fox News that telling people they're on the list would “compromise security.” 

“If terrorists knew they were on the list, they would change their identity or turn to an operative who may not be on the list to conduct an attack,” the TSC official said.

Jamal Tarhuni is one of the Americans on the no-fly list, and he says the feds got it wrong. The Oregon businessman only learned he was on the list when he tried to fly home to Portland from Tunisia. He had just completed a humanitarian visit to his native Libya with the Christian group Medical Teams International. They delivered water and medical supplies to victims of the country's civil war.

Tarhuni says he was eventually interviewed for several hours by FBI agents and kept in Tunis for a month before being allowed to fly home. He remains on the no-fly list and still doesn't know why. He was recently forced to take a 36-hour train ride to give a speech in Minneapolis. Then he drove 17 hours to Washington, D.C., to meet with the Libyan prime minister. Altogether his travel took more than 100 hours.

“I think it's my right, as an American citizen, that whoever placed me on the no-fly list should come out and tell me why,” says Tarhuni.

There is a redress procedure that allows people who are mistakenly put on the list to have their name removed. Changes to the list are made daily. Jamal Tarhuni has not attempted redress, saying his lawyer believes it requires that he give up some of his constitutional rights. Tarhuni claims the FBI wanted him to take a polygraph test in Tunis, but he refused because he would have had to sign away his Miranda rights.

“They can ask me anything they want about what I know, and I'll be more than happy to answer, because I have nothing to hide about what I've done in Libya, or how I've lived my life in the United States,” says Tarhuni.

The FBI says to be put on the no-fly list a person must be on the larger terrorist watch list and be considered a threat to the aircraft, its passengers or to national security. Jamal Tarhuni freely admits he worships at the Islamic Center of Portland, a mosque that has been at the center of several terrorist investigations since the terror attacks of Sept. 11, 2001.

The Portland Seven, a group of jihadists who tried to enter Afghanistan to fight American soldiers, belonged to the Islamic Center of Portland. It's imam, Sheik Mohamed Kariye, was accused by one of the Portland Seven of funding the effort. Kariye was never charged with terrorism, but is also on the no-fly list and is one of the plaintiffs in the ACLU case.

Mohamed Osman Muhamud, a Somali-born U.S. citizen who was arrested for trying to detonate a fake van bomb at a Portland Christmas tree lighting ceremony in 2010, also attended the mosque.

A former member of the Portland Joint Terrorism Task Force says the no-fly list has saved lives. “I don't think the mosque is any threat,” says Retired Milwaukie (OR) Police Chief Larry Kanzler. “It's the activities of the people who attend that location who have drawn attention to themselves by their activities.”

Jamal Tarhuni says he never met any of the Portland Seven or Muhamud. And as far as his trips to Libya, he says ever since he fled the country, he has protested against the Qaddafi regime.

He hopes to be allowed to fly back in June to work with the new government in establishing a democracy that serves all the people.