In the aftermath of the Orlando, Fla., terrorist attack, the fact that Omar Mateen was twice investigated by the FBI has many wondering why he was allowed to purchase firearms and why the FBI wasn't aware of his plans for terror. Here are the key details about America's terror watch lists and what they do.

WHAT IS THE TERRORIST WATCH LIST?

The Terrorism Screening Database is the official name for the terrorist watch list and is maintained by the FBI's Terrorist Screening Center. The Terrorism Screening Database is the country's central repository of foreign and domestic known and suspected terrorists.

It receives names of suspected international terrorists from the Terrorist Identities Datamart Environment, which is maintained by the National Counterterrorism Center in connection with the U.S. intelligence community and security agencies that have information on terrorists. It also receives data on domestic terrorists from the FBI.

The Terrorism Screening Database only includes information used to identify terrorists. The database itself does not include classified information on terrorists regarding what they have done and how we have been tracking them. For foreign terrorists, this classified information is maintained in the Terrorist Identities Datamart Environment, while for domestic terrorists, it's kept by the FBI.

HOW IS THE TERRORISM SCREENING DATABASE USED?

From the Terrorism Screening Database, more specific lists are created for different purposes. For example, the No Fly and Selectee lists are used to prevent individuals from travelling or to subject them to greater scrutiny. For an individual to be included on the No Fly or Selectee list, additional evidence of his threat to aviation security and clear identifying information is needed above and beyond the reasonable suspicion standard.

Another list extracted from the Terrorism Screening Database is the Known and Appropriately Suspected, or KST file. To be included on the KST file, clear identifying information is needed. The KST is queried by the National Instant Criminal Background Check System that is used to check firearm purchases. Inclusion on the file does not itself prevent a gun purchase, but flags the purchase for further review. The individual is allowed to purchase a firearm so long as he is not a felon, adjudicated to be mentally ill, a fugitive from justice, an illegal immigrant, or prohibited from making gun purchases for another statutory reason.

WHAT IS THE STANDARD OF PROOF NEEDED TO BE PUT ON THE TERRORISM SCREENING DATABASE?

To get on the Terrorism Screening Database, U.S. officials nominate an individual whom they have "reasonable suspicion" to believe is engaged in or aiding terrorist activities. There must also be a sufficient level of identifying information to include an individual on the list. There have been, and continue to be, legal battles over whether proper avenues for redress exist to get off the No Fly list.

SO HOW DID OMAR MATEEN GET HIS GUNS?

Omar Mateen was subject to two FBI investigations in 2013 and 2014 and was added to the Terrorism Screening Database. Following the conclusion of the investigations, Mateen was removed from the Terrorism Screening Database. As a result, when he went to purchase the firearms used in the attack, he was not in the KST file and was not flagged by the FBI. Even if he had been on the list, he would likely not have been prevented from purchasing a weapon. The FBI would have been alerted, however, and could have responded and investigated further.

DID THE FBI MAKE A MISTAKE?

The FBI's decision to close its investigations will be reviewed. FBI Director James Comey has said that it appears the bureau followed the correct procedures, but the steps the agents took are being examined to see if something fell through the cracks.

Until we know more about how the FBI investigated Mateen, it is too early to assume the FBI did something wrong. It does, however, point to the critical role the FBI plays in investigating and preventing terrorism and why our intelligence and law enforcement agencies need the right resources and tools, as well as the right coordination with state and local law enforcement, to stop terrorists before they strike.