In 2010, an American security contractor attempting to board a plane to Texas from Bogotá, Colombia, was denied a boarding pass. That was when Raymond Earl Knaeble first learned that he had been placed on a U.S. government terrorist watch list.

Knaeble, who was heading home before starting a new job in Qatar, was delayed for weeks. He lost the job. He later sued the Justice Department with the help of the American Civil Liberties Union, claiming his due process rights were violated.

Omar Mateen was also on a government terrorist watch list — on two occasions — but had been taken off by the time he bought the guns he used to massacre 49 people at an Orlando nightclub.

Critics have long blasted the effectiveness and reliability of the terrorist watch list system, asserting that too many people, like Knaeble, are swept up by law enforcement agencies without proper vetting or due process — and that too many other, certifiable threats, like Mateen, are left out.

Those concerns received heightened attention in the past few weeks, as the U.S. Senate considered, and ultimately rejected, proposals aimed at expanding a ban on gun sales to people on government terrorist watch lists. A sit-in by Democrats in the House of Representatives seeking to force a votes on the watch list and other gun restriction bills ended with lawmakers vowing to continue their campaign after the Independence Day holiday.

That debate has scrambled traditional alliances, with Democrats who have spoken out against government surveillance pushing to impose a ban on those who appear on the watch list from buying guns, and the ACLU and the National Rifle Association decrying such a move as a civil rights violation.

Here is a guide to understanding the debate as it moves forward.

How big is the terrorist watch list? Who is on it?

What is often described as a single, unified list is actually a collection of lists, or databases, created or expanded under the George W. Bush administration to avoid the breakdowns in intelligence sharing that precipitated the September 11 terrorist attacks.

Here’s how it breaks down:

The FBI’s Terrorist Screening Database is the U.S. government’s primary watch list, and what people usually mean when they refer to the terrorist watch list. It contains about 1 million records. About 5,000 of those records — 0.5 percent — are about Americans.

The screening database is subdivided into other lists. The most well-known of these is the no-fly list, whose members are barred from commercial air travel that passes through the United States. The no-fly list contains about 81,000 names, up from 16 on the eve of September 11, 2001. About 1,000 are Americans.

Though members of the no-fly list are not technically barred from entering the United States by other means, that often doesn’t matter, as those who attempt to cross the border via land or sea are often detained and interrogated along their journeys. The FBI shares watch list information with other countries, and U.S. Customs and Border Protection gives recommendations to ship captains about passengers who may pose risks to transportation security.

This sharing of watch list information ensnared Knaeble as he sought to return to the U.S. from Colombia. In May 2010, after a two-month delay, he flew to Mexico City, planning to cross into the United States by land. After he touched down, Mexican authorities detained him at the gate, questioned him for three hours, barred him from traveling farther into Mexico, and flew him back to Bogotá.

Are watch lists new?

The FBI has blacklisted suspected threats almost since its inception. J. Edgar Hoover infamously amassed a list of “enemies of the United States” that included terrorists, communists, spies, anti-war protestors, and civil rights leaders.

The modern terrorist watch list was created after the September 11 attacks. At that time, various government agencies maintained about a dozen separate lists with names of suspected terrorists. After the attacks, the law enforcement and intelligence communities were roundly criticized for failing to share important information, and the consolidated watch list was created to avert future communication blunders.

Can people in the FBI’s terrorist screening database buy guns?

Yes. Prospective gun buyers can only be denied if they fall into one of nine categories of purchasers prohibited under federal law — felons and drug users, for example. Being a suspected terrorist isn’t one of them. The Government Accountability Office reported that between 2004 and 2015, known or suspected terrorists tried to buy guns 2,477 times — and 91 percent of their background checks were approved.

It seems like a no-brainer that if someone’s a suspected terrorist, they shouldn’t be allowed to purchase a firearm. So what’s the deal?

Critics say the criteria for designating terrorists have grown so broad and ambiguous that anyone can get caught in the government’s dragnet. They point to several episodes when Americans were misidentified as belonging to the no-fly list, including, most memorably, the late Senator Ted Kennedy and musician Yusuf Islam, also known as Cat Stevens.

Last year, the The Intercept published a government document that spelled out the process for putting suspects in the terrorist database. The guidelines say agencies can nominate candidates for the list if there is “reasonable suspicion” to believe they are a “known or suspected terrorist.” That’s a relatively low bar, and the guidelines even make clear that agencies don’t need “concrete facts” or “irrefutable evidence” to back up their assertions. The guidelines also give a single White House official unilateral authority to place entire categories of people on the no-fly list.

When gun rights enter the debate, the stakes become higher. Opponents believe that barring people on the list from buying guns is like punishing them before they have been convicted of a crime. Gun-rights groups, including the NRA, have expressed support for a proposal introduced by Republican Senator John Cornyn of Texas that would give the Justice Department three business days to prove that a possible extremist was likely to commit terrorism before a gun sale could go through.

So what’s the point of having a list that can be so inaccurate?

It’s difficult to measure how successful the watch list has been at preventing terror attacks. The Washington Post reported in 2007 that few of those flagged by the list were arrested or denied entry into the United States. But Leonard Boyle, then-director of the FBI branch overseeing the watch list, said using arrests as a criterion “misperceives the role” of the list in combating terrorism. Even when authorities don’t have enough evidence to make an arrest, watchlisted individuals can be denied visas, licenses to drive hazardous materials or, as in Knaeble’s case, boarding of airplanes.

Every day, Boyle wrote, the watch list “results in known and suspected terrorists being denied entry into this country, refused boarding on commercial flights, or identified by state and local law enforcement officers during routine stops,” making Americans “significantly safer.”

But critics can point to several examples of how the watch list system failed to prevent an attack. Some recent terrorists were either on a watch list at some time — like Tamerlan Tsarnaev, who successfully bombed the Boston marathon — or never at all — like the couple who killed 14 people in San Bernardino, California, last year.

The FBI added Omar Mateen, the Orlando shooter, to the list in 2013 and 2014 as agents investigated his inflammatory remarks to coworkers and for his apparently tenuous connections to a suicide bomber in Syria. But Mateen was removed from the list after the FBI couldn’t find enough evidence to warrant a deeper investigation. Earlier this month, Mateen walked into a gay nightclub in Orlando and shot dead 49 people.

Critics say the growth of the watch lists over the years has made them unwieldy and diverted precious resources into investigations of suspects who likely have no links to terrorism.

“It has low sensitivity and a very low specificity, meaning it’s been totally useless,” Marc Sageman, a former CIA case officer who wrote the upcoming book Misunderstanding Terrorism, tells The Trace.

Do names get removed from the list?

Yes. The government has deleted thousands of records from its system over the last five years.

How do I know if I’m on the watch list?

You probably won’t ever know for sure if you’re on the master watch list. The FBI doesn’t disclose its contents. But it’s a good bet that if the bureau visits you about a terrorism-related investigation, you’re probably on there.

As a result of the ACLU’s lawsuit with Knaeble, the government in 2015 began telling Americans they were on the no-fly list and offering some reasons. Travelers still don’t get advance notice, however, so the only way to find out if you can’t fly is to try to board a plane and see whether you get accosted by security. People on the list can file for redress. But Hugh Handeyside, a staff attorney with the ACLU’s National Security Project, says the government provides scant information and there is still no process for contesting your inclusion in front of a neutral third-party.

Knaeble, the U.S. Army veteran stranded in Bogotá, was eventually able to return to the United States in August 2010 — but not without trouble. He traveled for 12 days, passing through Panama City before arriving in Mexicali, California. Along the way, American and foreign authorities detained, interrogated, and searched him numerous times.

Even though the government has changed its policies about alerting travelers on the no-fly list, the ACLU contends that the process is still far short of constitutional and is fighting in court to get it changed. The debate continues.

[Photo: Flickr user M&R Glasgow]