Dozens of people on a terrorist watch list were in Washington for pro-Trump events Jan. 6, a day that ended in a chaotic crime rampage when a violent mob stormed the U.S. Capitol, according to people familiar with evidence gathered in the FBI’s investigation.

The majority of the watch-listed individuals in Washington that day are suspected white supremacists whose past conduct so alarmed investigators that their names had been previously entered into the national Terrorist Screening Database, or TSDB, a massive set of names flagged as potential security risks, these people said. The watch list is larger and separate from the “no-fly” list the government maintains to prevent terrorism suspects from boarding airplanes, and those listed are not automatically barred from any public or commercial spaces, current and former officials said.

The presence of so many watch-listed individuals in one place — without more robust security measures to protect the public — is another example of the intelligence failures preceding last week’s fatal assault that sent lawmakers running for their lives, some current and former law enforcement officials argued. The revelation follows a Washington Post report earlier this week detailing the FBI’s failure to act aggressively on an internal intelligence report of Internet discussions about plans to attack Congress, smash windows, break down doors and “get violent . . . go there ready for war.”

Other current and former officials said the presence of those individuals is an unsurprising consequence of having thousands of fervent Trump supporters gathered for what was billed as a final chance to voice opposition to Joe Biden’s certification as the next president. Still, the revelation underscores the limitations of such watch lists. Although they are meant to improve information gathering and sharing among investigative agencies, they are far from a foolproof means of detecting threats ahead of time.

Since its creation, the terrorist watch list, which is maintained by the FBI, has grown to include hundreds of thousands of names. Placing someone’s name on the watch list does not mean they will be watched all of the time, or even much of the time, for reasons of both practicality and fairness, but it can alert different parts of the government, such as border agents or state police, to look more closely at certain individuals they encounter.