One child has died and another is missing after the two 10-year-olds went into a rain-swollen river while playing in a park in Passaic, New Jersey, on Wednesday, authorities said.

An 11-year-old survived, the sister of the 10-year-old boy who drowned.

“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,” Passaic Mayor Hector Lora told reporters Thursday. As she went underwater, she felt rocks, Lora said.

The other 10-year-old, a girl, was still missing early Thursday. Multiple boats and divers searched an expanded area of the river, using drones and sonar technology, Passaic Fire Department Chief Patrick Trentacost said in an email.

Trentacost referred to the search as a “recovery operation,” saying it will continue “as long as necessary.”

No names have been released.

The three children were seen in the Passaic River in Pulaski Park. It wasn't immediately clear how they ended up in the river, which was higher than usual because of heavy rainfall in the region, police in neighboring Paterson said. Firefighters in that community rescued about 20 people who became trapped by rapidly rising floodwaters.

Trentacost said they “quickly slipped deeper due to the fast current and high water level.”

Lora said first responders got calls at about 6:20 p.m. and a man in the park tried to rescue the children before they arrived.

“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,” Lora said. “The gentleman expressed to us his obvious pain and hurt.”

The river flows about 80 miles (129 kilometers) through forest land and suburbs before entering more urban areas and emptying into Newark Bay. Passaic is about 10 miles (16 kilometers) north of New York City.