WASHINGTON, D.C. (AP) — A fugitive task force attempting to serve a warrant Monday in Charlotte, North Carolina, ended in the deadliest day for U.S. law enforcement in a single incident since 2016.

Four officers were killed and four more were wounded after the suspect began firing from an upstairs window, starting a shootout with task force officers. More officers responded as backup and were also struck by gunfire, authorities said.

At least three officers on the initial task force were killed — a deputy U.S. Marshal and two officers with the North Carolina Department of Adult Correction — as well as an officer with the Charlotte-Mecklenburg Police Department.

The task force is one of dozens led by the US Marshals that find fugitives and serve hundreds of warrants every day around the U.S. Here’s a look at how they operate and how they take on the task of finding and arresting some of the most dangerous suspects:

WHAT ARE TASK FORCES?

Federal law enforcement agencies often work with local police around the country on task forces, which are typically teams of law enforcement officers from different agencies working together on investigations in a specific area such as drugs, terrorism or gangs.

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The U.S. Marshals Service is the primary federal agency tasked with tracking down fugitives, and their task forces are typically focused on finding people wanted on criminal warrants. They also often take the lead when searching for a known suspect in a high-profile crime or finding escaped prisoners.

There are nearly 4,000 deputy U.S. Marshals, and they work with more than 3,500 officers from departments around the country on dozens of fugitive task forces including local and regional efforts. It’s a setup that can benefit both sides: local police bring manpower and deep knowledge of an area, while federal law enforcement has access to additional resources. Working with federal law enforcement can also bring expanded authority and more serious federal charges to the table.

HOW DO THEY WORK?

An assignment to a federal task force is often a full-time job that requires rigorous training and qualifications, said Mark Lomax, a consultant and former executive director of the National Tactical Officers Association.

When Lomax served as a major in the Pennsylvania State Police, he had several troopers assigned to federal task forces, including with the U.S. Marshals.

“They are very, very professional. This is what they do. When you are going after bad guys, really, really bad characters that’s who you want there, the marshals,” Lomax said. “They are trained to do it. They are equipped to do it. They have the mentality. They are the best of the best. And that includes the task force members. They pick individuals who meet their requirements, and they go through a lot of training and are matched with personnel.”

Officers are usually assigned to federal task forces for at least a couple years, though the duration varies by location and agency, said Anthony Cangelosi, a lecturer at John Jay College of Criminal Justice and former deputy U.S. Marshal in New York City.

HOW DO THEY CATCH FUGITIVES?

U.S. Marshal task forces are usually looking for people who have been charged with a crime, but are out on the run or have failed to appear in court.

Investigators are looking for last known addresses, their friends and family, credit card usage — anything that might point to where they are, Cangelosi said.

“You’re just trying to find anything that could locate that individual,” he said.

Operations like the warrant being served in Charlotte don’t happen without planning, Lomax said. Reconnaissance and surveillance is a large part of the work. When task force members are sure they have found a viable location for a fugitive, there’s sometimes even practice drills.

“They are working informants. They are doing their due diligence, talking to family members, friends, and nowadays there’s a lot of technology involved,” he said. “Once they have identified where the individual may be, they will surveil and do intel and have sufficient personnel and equipment to be able to address that situation.”

Lomax said that may involve watching a house to see if there are children or dogs or other factors that need to be considered, especially when there’s a weapon involved.

“They may even go to a parking lot or other area and practice how they plan to enter or approach, figure out the best approaches,” he said. “They do a lot of homework. But no matter how much planning and preparation you do, you don’t know what the person on the other side of that door is going to do. Even if you know they could be armed and you prepare, you can’t prepare for everything.”

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Lauer reported from Philadelphia.