LEWISTON — Divers concluded their search of the Androscoggin River on Friday for a 5-year-old boy who plunged into the chilly waters Tuesday evening.

“Diver search options for the river have been exhausted,” Auburn Deputy Chief Jason Moen wrote in an email Friday afternoon.

He said the Maine Warden Service would continue with boat and aircraft patrols as weather permits.

He said the boy’s 9-year-old brother, who had jumped in after the 5-year-old, had improved from critical to stable condition at Maine Medical Center in Portland.

Officials from the Warden Service said Friday that the Androscoggin River’s increasingly swift currents were hampering their search in the recovery of the 5-year-old boy.

Conditions on the river made it too dangerous for divers to search the bottom, so the dive team deployed sonar equipment to aid in their search, Lt. Adam Gormley said.

“Obviously, at day three, we’re not giving up,” he said. “We still want to locate this little boy. But it’s environmentally challenging.”

The river’s water volume is three times greater than it was when the search began, Gormley said.

“That is ripping,” Sgt. Bruce Loring said, pointing to chunks of ice in the river moving at 3 to 4 mph.

Dam operators upriver have worked with divers to try to regulate water flow, Moen said.

Three boats were launched Friday off Lincoln Street, along with seven divers.

The search was to continue from the spot where the boy entered the river and move downriver about 1 mile, said Loring, who is in charge of the dive team.

If the sonar indicated an object under the water consistent with the boy’s body, the spot would be marked with a buoy and divers would search that area as water conditions allowed, he said.

“At this point, it’s undiveable,” Loring said. In addition to the strong current, there was “zero visibility” on the bottom, he said. “You literally cannot see your hand in  front of your face.”

The sonar deployed on the boat Friday sends out sound waves that are read by a laptop computer on board the boat.

“That’s our best tool right now,” he said.

Aircraft and boats have scoured the surface of the river and banks for any sign of the boy all the way to the first dam in Lisbon, Gormley said. Pilots have flown as far as Merrymeeting Bay.

Gormley said there’s no way to tell whether the boy went to the bottom in the search area or “floated all the way down” the river.

“In the spring, there’s just so much water, it’s just a free-flow right to the ocean,” he said. “We’re hoping that’s not the case.”

Gormley urged people to exercise extreme caution when approaching the river and getting close to the water to look. If they see something, they should call 911.

“This water’s cold, it’s moving fast and it’s unpredictable,” he said. “The banks are muddy, they’re steep and they’re full of debris. We don’t need another fatality.”

In 45-degree water, hypothermia is likely to set in within 10 to 15 minutes, Gormley said. “They’re in trouble, big trouble.”

Gormley said the search would continue by conducting constant dam checks, routine boat patrols and frequent flights up and down the river.

“By far, we’re not giving up,” he said. “I guess that’s the biggest message.”

cwilliams@sunjournal.com