Certain Americans still won't be able to board commercial airplanes because officials fear they represent a terrorist threat, but soon they'll know the specific reasons why.

The US government has agreed on 15 April to share information with targeted passengers that explains the rationale behind the determination to place them on a "no-fly list". Barred passengers will also have the opportunity to challenge their no-fly status.

The decision follows a court victory by the American Civil Liberties Union, which filed a lawsuit against the government on behalf of 13 US citizens. The government modified its protocol after a judge ruled in June 2014 that the old policy violated the Fifth Amendment's guarantee of due process under the Constitution.

"The US government is making enhancements to the Department of Homeland Security Traveller Redress Inquiry Program to provide additional transparency and process for US citizens and lawful permanent residents who have been denied boarding on a commercial aircraft because they are on the No Fly List," the department said in a statement.

Under the revised procedure, barred travellers can request more details after initial information. The government will then prepare a second, more thorough response, identifying "specific criterion under which the individual has been placed on the No Fly List," assuming the information is "consistent with the national security and law enforcement," said court documents filed by the Justice Department in Oregon and Virginia.

Passengers will then have the opportunity to dispute their status in writing, with supporting information, and will receive a final written determination from the Transportation Security Administration under the new protocol.

ACLU officials praised the changes — to a point. "It's good that the government is finally going to tell people of their status on the no-fly list," Hina Shamsi, the lead attorney in the case, said in a statement to CNN.

"Unfortunately, the government's new redress process falls far short of constitutional requirements because it denies our clients meaningful notice, evidence and a hearing." Shamsi said the ACLU is challenging the new protocol in court.

The number of people on the list — which is maintained by the Federal Bureau of Investigation's Terrorist Screening Center — is classified, but estimates have been at least 21,000, including 500 Americans.