US terror alert system, explained

A Homeland Security Vehicle is pictured in this file photo. The current United States terror alert system, known as the National Terrorism Advisory System (NTAS), issues their warnings as “alerts” and “bulletins.” (REUTERS/Mike Blake)

British Prime Minister Theresa May announced this week her country's threat level would be raised to 'Critical' for the time being, after the deadly suicide bombing at the Ariana Grande concert in Manchester.

The current United States terror alert system, known as the National Terrorism Advisory System (NTAS), issues their warnings as “alerts” and “bulletins.” The Homeland Security Advisory System (HSAC), which featured different colors to indicate threat level severity, is no longer used.

Here's how the NTAS works:

Alerts

These are issued "when there is specific, credible information about a terrorist threat,” the Department of Homeland Security (DHS) says on their website.

Elevated alerts are issued when there is "only general information about timing and target," according to the department. Imminent alerts, meanwhile, are for threats thought to be “credible, specific, and impending in the very near term."

Alerts can feature the area, transportation method, or infrastructure that could be affected, the DHS says, and possible safety measures.

UK TERROR THREAT LEVELS: WHAT DO THEY MEAN?

Bulletins

These are a step down from alerts, and offer "broader or more general information about terrorism trends, events, and potential threats" when there isn't a specific or credible threat toward the U.S., the DHS says.

They can explain what the issue is, ways the U.S. is fighting terrorism, and how people can help fight terrorism, according to the department.

Determining what type of advisory to issue

The DHS says it decides to send out NTAS advisories by considering intelligence assessments, as well as "risks to the public and critical infrastructure."