Since the mass shooting in a gay nightclub in Orlando on June 12, Democrats have endorsed various measures to get weapons out of the hands of people on secretive terror watch lists. (Rich Pedroncelli/AP)

The push by congressional Democrats to bar suspected terrorists from acquiring guns and explosives has focused renewed attention on the government’s secretive terrorist watch lists, which have grown exponentially since the 9/11 attacks and triggered widespread concern among civil liberties groups.

Since the mass shooting in a gay nightclub in Orlando on June 12, Democrats have endorsed various measures to get weapons out of the hands of people on the lists. The Senate is expected to vote Monday on a series of competing gun-control measures that will highlight the continuing partisan divide over the issue.

The Orlando shooter, Omar Mateen, had been on the FBI’s terrorist watch list but was removed in 2014. His was one of approximately 800,000 names in that database, the most prominent of at least seven overlapping watch lists maintained by at least four federal agencies.

The system has grown so large and complex that Sen. Bob Corker (R-Tenn.), the chairman of the Senate Foreign Relations Committee, said this week that the Senate will vote “with almost no one understanding how these lists are put together, how they’re adjudicated, how you get off of them.” He added: “The terror watch list is not what people think it is . . . and there’s 11 different lists.”

Although the government does not release the exact number of watch lists or the specific criteria for getting on them, it is clear that they have exploded in numbers. The no-fly list — once called the “no transport” list — contained 16 people on Sept. 11, 2001, according to government documents obtained by the ACLU of Northern California. By 2014, it had grown to about 64,000 people, FBI officials said.

Federal terrorism investigators say the lists are an essential part of their post-9/11 emphasis on preventing another mass attack. Some experts agree.

“We want to give sufficient tools to authorities to keep us safe while at the same time building in safeguards,’’ said Bruce Hoffman, director of national security studies at Georgetown University. “It’s one of the strengths of our society that if you haven’t committed a crime, you can’t be arrested. But that doesn’t mean someone is, quote unquote, innocent.’’

Civil liberties advocates strongly dispute that, saying the watch lists are riddled with inaccurate and outdated information, nearly impossible to get off and stigmatize the people on them, who are stopped at airports and border crossings and rarely told why. “The government’s bloated watch system is based on vague and overbroad standards,’’ said Hina Shamsi, director of the American Civil Liberties Union’s national security project. “As a result, innocent people are wrongly blacklisted without a fair process to correct the error.’’

The largest watch list is The Terrorist Identities Datamart Environment (TIDE), maintained by the National Counterterrorism Center. It is a central repository of not only names of what the government calls “known or suspected terrorists’’ but also classified intelligence files on them. As of August 2014, it contained about 1.1 million names, including about 25,000 U.S. citizens and legal permanent residents, according to the center.

The center sends data from TIDE to the FBI’s Terrorist Screening Center, which maintains what is known as the government’s “consolidated Terrorist Watchlist.’’ It contains about 800,000 names, according to 2014 congressional testimony from Christopher M. Piehota, the screening center’s director.

He told a House subcommittee that the watch list “contains both known or suspected international and domestic terrorist identity information.’’ The process for getting on the watch list, he said, is known as “nominations,’’ which originate from federal law enforcement, homeland security and intelligence agencies.

Information from the FBI’s master list flows to other lists, including the no-fly list, which is also maintained by the FBI; the Consular Lookout and Support System, a State Department list that identifies people who may be ineligible for a visa or passport; and the Known or Appropriately Suspected Terrorists File. That list, also maintained by the FBI, is a subset of the bureau’s master watch list and is disseminated nationwide to law enforcement agencies, according to a 2014 ACLU report.

In the years since 9/11, government monitors and courts have documented problems with the expanding system. A 2009 Justice Department inspector general report said that the FBI was often failing to modify information about individuals on its master watch list when new information emerged during an investigation.

In 2014, a Malaysian architecture professor prevailed in a lawsuit in which she sought to get her name removed from the no-fly list, with a federal judge saying she had been put on it by mistake.

And last year, another federal judge ruled that the government’s lack of effective procedures for people to challenge their inclusion on the no-fly list was unconstitutional. The government then said that U.S. citizens and residents, unlike before, can find out whether they are on the list and sometimes receive a summary of the reasons. The ACLU, which filed the lawsuit, contends that the revisions are insufficient.

Karoun Demirjian contributed to this report.