WASHINGTON  The Federal Bureau of Investigation has incorrectly kept nearly 24,000 people on a terrorist watch list on the basis of outdated or sometimes irrelevant information, while missing people with genuine ties to terrorism who should have been on the list, according to a Justice Department report released Wednesday.

The report said the mistakes posed a risk to national security, because of the failure to flag actual terrorism suspects, and an unnecessary nuisance for nonsuspects who may be questioned at traffic stops or kept from boarding airplanes.

By the beginning of 2009, the report said, this consolidated government watch list comprised about 400,000 people, recorded as 1.1 million names and aliases, an exponential growth from the days before the attacks of Sept. 11, 2001.

Among the list’s uses is the screening of people entering the country, and intelligence officials say it has allowed agencies to work together to prevent the type of breakdown that allowed two of the Sept. 11 hijackers to get into the United States even though they were known to the Central Intelligence Agency for their terrorist ties.