A desperate search is underway for a 10-year-old girl after three children were swept away by a rain-swollen river in Passaic, New Jersey.

One 10-year-old boy drowned while his 11-year-old sister survived the harrowing ordeal on Wednesday. Passaic Mayor Hector Lora said the boy's final actions may have saved his sister's life.

"He put his hand on her head, trying to get up above the water, and he may have inadvertently saved her life without even knowing," Mayor Lora told reporters on Thursday. “She described to me as she was going into the water he placed his hand on her head as he was trying to go up, and she ended up feeling the rocks where she was at and climbed out because she couldn’t see him.”

The search for the other 10-year-old girl continued into Thursday morning.

Passaic Fire Department Chief Patrick Trentacost said multiple boats, divers, drones and sonar technology were being used to try and find the girl. Chief Trentacost described the efforts as a "recovery operation," vowing it would continue "as long as necessary." No names have been released by authorities.

The three children were last seen in the Passaic River within Pulaski Park. Police in neighboring Paterson noted the river's unusually high level due to recent heavy rainfall, though it remains unclear how they entered the water.

Chief Trentacost stated they "quickly slipped deeper due to the fast current and high water level."

First responders received calls around 6:20 p.m., and a man in the park made a valiant but unsuccessful attempt to rescue them.

"He went in about 5 meters deep (16.4 feet) and when he got too deep and the current was pushing him away, he could not reach them,” Mayor Lora recounted. “The gentleman expressed to us his obvious pain and hurt.”

The Passaic River flows approximately 80 miles through forest land and suburbs before entering more urban areas and emptying into Newark Bay. Passaic is situated about 10 miles north of New York City.

Firefighters in Paterson also rescued about 20 individuals trapped by rapidly rising floodwaters in the region.