WASHINGTON -- The Orlando massacre is fueling demands that the government greatly expand the number of people it monitors as potential terrorists, a proposal that critics worry veers dangerously close to the dystopian science fiction of "Minority Report."

While the ACLU and the NRA say it's already too hard for innocent people to get their names off watch lists, there are calls for the government to create an alumni list to track those whose investigations are closed for lack of evidence. The Orlando shooter, Omar Mateen, was removed from the government's list in 2014, so no one in law enforcement was alerted when he purchased firearms used in his attack.

"The terror watch list is an on-off switch," said Juliette Kayyem, a former assistant secretary of Homeland Security. "We need to think about gradations of it, especially when it comes to purchasing weapons and travel."

The calls for broader surveillance come as the challenge of identifying potential threats is vastly complicated by the shift from the centrally directed terrorism of al-Qaida to the individual "lone-wolf" attacks that Islamic State seeks to inspire. A task that once relied heavily on identifying connections to known terror groups and targeting weak links in a conspiracy now depends instead on the vagaries of determining an individual's evolving psyche.

An official with the American Civil Liberties Union a year ago compared the opaque process of compiling the watch list to the "precrime" policing of "Minority Report." In the 1956 short story by Philip K. Dick and a 2002 movie starring Tom Cruise, a fictional society drives down crime rates by acting on predictions of citizens' future conduct.

The government's watch list is already larger than the population of the nation's capital.

Though the government doesn't release information on the current size of the list, it included 800,000 people in September 2014, according to congressional testimony. U.S. citizens or permanent residents made up 5 percent of the list at the time of a Congressional Research Service Report in 2013.

That implies the government assessed about 40,000 Americans and 760,000 foreigners as presenting a greater threat than Mateen before the Orlando attack, assuming there haven't been major changes in the size of the list or the proportion of Americans on it.

The FBI's Terrorist Screening Center, which maintains the watch list, says it adds people based on "reasonable suspicion" that they are engaged in, preparing or aiding terrorist activity, or have done so in the past.

Based on the information that has become public about the FBI investigation, Mateen wouldn't have met the standard of "reasonable suspicion" of terrorist activity or preparations at the time, said Kayyem, now a lecturer at Harvard's Kennedy School of Government and author of "Security Mom: An Unclassified Guide to Protecting Our Homeland and Your Home."

Sen. Susan Collins of Maine, a Republican on the Intelligence Committee, said someone like Mateen raised enough red flags that he should have remained on the list anyway.

Dianne Feinstein of California, the top-ranking Democrat on the Senate Intelligence Committee, this week proposed a new version of a bill to ban gun purchases by people on the terror watch list to also include anyone who has been investigated in the previous five years, even if they were cleared. Democrats staged a marathon session of speeches on the Senate floor Wednesday and threatened to hold up a spending bill to force consideration of Feinstein's legislation.

Currently, suspected terrorists on the watch list are allowed to buy guns, but the intelligence agency that placed them on the list is notified when they make a purchase. That is provided the purchase is made through an authorized dealer who performs a background check rather than at a gun show or through a deal between individuals.

Last year people on the watch list were involved in 244 attempts to buy guns from authorized dealers, of which 223 were allowed to proceed and 21 were denied, according to data from the Government Accountability Office released by Feinstein on Wednesday.

Some law enforcement officials worry that more expansive monitoring will place an undue burden on an agency that already is struggling to weed out potential terrorists. The FBI initiates thousands of counterterrorism cases every year.

The agency currently has 7,000 to 8,000 pending counterterrorism cases, which includes full investigations and preliminary investigations, said one law enforcement official who insisted on anonymity. Mateen's case never moved beyond a preliminary investigation. About 1,000 of those cases are related to homegrown violent extremism across the country, the majority of which are connected in some way to Islamic State, two officials said.

Sometimes people are investigated based on bogus information, such as a tip from an estranged spouse or partner with a vendetta, the second official said. Creating a new system under which agents would be notified if someone who was previously cleared of wrongdoing tries to buy a gun would be burdensome and potentially a violation of rights, the official added.

The Federal Bureau of Investigation has about 13,500 agents working worldwide whose top mission is counterterrorism. But the number of agents working those cases will ebb and flow depending on priorities, and investigators have other vital missions including cybersecurity cases and white-collar crimes.

The FBI opened a preliminary investigation into Mateen that lasted 10 months, from May 2013 to March 2014, after coworkers in his job as a contract security guard at a courthouse raised concerns that he had terrorist leanings. Agents found no evidence to challenge his claim that he made statements about terrorist connections "because he thought his coworkers were discriminating against him and teasing him because he was Muslim," FBI Director James Comey told reporters this week.

Agents questioned Mateen again two months later because they learned he had a brief association with Moner Mohammad Abusalha, the first American suicide bomber to die in Syria. Agents concluded the contact was minimal and the inquiry never led to a new investigation.

"That's a picture of someone who warrants to be left indefinitely on the list, unless there is exculpatory information," Collins said. "We're just talking about reason to believe that he could be a danger to our country at some point."

Yet that judgment is especially difficult for counterterrorism agents to make regarding lone actors who don't conspire with terrorist groups and may not have made any preparations, Kayyem said.

"The quintessential question is: Can any safety and security apparatus predict from one, two, three interviews whether that person will be the one who goes through the front door of a bar and kills someone?" Kayyem said. "I just don't know that we have the capacity to do that."

The watch list already relies on "vague and over-broad standards" in which Americans can't confront evidence against them kept secret in the name of protecting intelligence sources and methods, said Hina Shamsi, director of the ACLU's National Security Project and author of the article on Slate comparing the process to "Minority Report."

"Predictive judgments guarantee a high risk of error," Shamsi said. "So if the government is going to have a system that predicts Americans might become a threat and blacklist them, that is even more of a reason for the fundamental due process safeguards that we seek and the government is refusing to provide."

The ACLU challenged the government's No-Fly list, which bars some people on the watch list from boarding flights, in a 2010 lawsuit that is still pending. Preliminary rulings already have required changes in the redress process. About 8 percent of those on the watch list are designated for No-Fly status, according to the 2014 congressional testimony.

Meanwhile, presumptive Republican nominee Donald Trump and the National Rifle Association indicated Wednesday that they may meet to discuss barring suspected terrorists from buying firearms. But the politically powerful organization said its long-held concerns about the watch list still stand.

"The NRA believes that terrorists should not be allowed to purchase or possess firearms, period," said Chris Cox, a lobbyist for the group. "At the same time, due process protections should be put in place that allow law-abiding Americans who are wrongly put on a watch list to be removed."

(With assistance from James Rowley, Steven T. Dennis and Sahil Kapur)