FBI’s Underwater Search and Evidence Response Team, Part 3

Despite the usual pitch-black environment they work in, divers rarely use lights below the surface because particulate matter churned up from walking on the bottom diffuses any light. Instead, divers often rely on intelligence gathered during the initial dive, as well as metal detectors and sonar equipment—usually controlled from the surface—to locate and direct them toward evidence and other items they’re searching for.

Sonar, short for “sound navigation and ranging,” is a technique that uses sound waves to locate objects. It is useful for exploring and mapping bodies of water because sound waves travel farther in the water than radar and light waves. “As items go up in size, the areas that need to be searched get bigger, and the initial intelligence about the site may not be quite as concise,” said Hudson, who explained that sonar systems are especially useful in these scenarios.  

USERT also uses remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), which both collect images and other data underwater, as well as investigate areas that are too dangerous for divers.  

As divers find items to recover, they often place smaller objects in airtight containers and then carry them back up to the surface. 

But if items are over 15 pounds, divers use lift bags. Smaller lift bags look similar to parachute-shaped balloons that divers can inflate underwater and attach to the items that need to be raised to the surface. Divers can inflate these bags using either a regulator or a low-pressure inflator hose. This allows the diver to adjust the buoyancy of the lift bag and to safely control the ascent. Larger lift bags can raise objects as large and heavy as 2,000-pound vehicles.
 
Upon collecting all the items at the scene, USERT follows chain of custody guidelines. Each agency—whether the FBI or a local law enforcement department—follows its own rigorous processes. Most items are sent to a lab for further examination. Depending on the amount of time spent underwater, lab technicians can then gather information as detailed as cell phone data or DNA samples.  

USERT: Tools of the Trade

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