(CNN) -- Divers searching in the murky Mississippi River for victims of the bridge collapse work in a hazardous world where a mistake can cost them their lives.
Underwater debris swirls the river's already powerful currents into whirlpools of fast-moving water that can endanger divers, authorities said.
And the water is a potentially lethal stew of fuel, broken glass, jagged car parts, and concrete and rebar from the fallen bridge.
"Conditions on the river are even more treacherous than yesterday," Hennepin County Sheriff Rich Stanek said Friday. "The divers will be taking extreme caution. We will be slow and methodical during our search operations today."
Chunks of debris have been falling from the remains of the interstate bridge, which fell into the river in Minneapolis on Wednesday, creating hazards from above as well, Stanek said.
In addition, there's a large power line in the area of the collapse, he said. "We do not want that to come down into the river with the rescue workers and all else that's going on." Watch veteran diver describe what Minneapolis rescuers face »
"This recovery operation is going to take a long time," Stanek said Thursday. "We are poised to be here for several days, if not longer."
Officials said there might be cars crushed under the main section of the collapsed bridge, which lies in the middle of the river. If there are, huge chunks of debris will have to lifted to reach them.
Stanek said the water level in the river was lowered a foot or two on Thursday, to about 7 feet deep, to make the search easier.
Visibility in parts of the river is less than a foot, said one diver, Capt. Shanna Hanson of the Minneapolis Fire Department.
In those conditions, groping inside the sunken cars is more important than looking in them, she said on CNN's Larry King Live Thursday night. "So, although you're trying to look and see what you can, you're also feeling all the spaces that a victim could have possibly gone into.
"There can be things underwater that can snag you that you can't see, currents that might change and shift, causing the debris to shift and pinching something in there," Hanson said.
At least five people were killed and 100 injured when Minnesota's busiest bridge inexplicably crumbled during Wednesday evening's rush hour, dumping dozens of cars into the river. The death toll is expected to rise as divers search the twisted debris, as well as areas upstream and downstream.
A veteran rescue diver, Pete Gannon, of the Plantation, Florida, Fire Department, said the divers in Minneapolis have a tough job ahead of them.
"They're going to get caught in all kinds of debris, you know, just trying to get out there," he told CNN's Anderson Cooper. "You saw the rebar hanging. Now, that rebar goes underwater. People don't realize, I'm going to swim into that and I'm in some kind of a cage. And I can't see it. You know, it's like swimming under a shopping cart." E-mail to a friend