(UPPER MAKEFIELD TOWNSHIP, Pa.) -- A search continued Monday afternoon for a 9-month-old boy and his 2-year-old sister after they and their mother were swept away in a flash flood in Bucks County, Pennsylvania, on Saturday while driving with other family members to a barbecue, officials said.
The search for the children was centered around Hough's Creek, just south of Upper Makefield Township, Chief Tim Brewer of the Upper Makefield Fire Company said at a news conference Monday.
"We ask everyone to keep the families in their thoughts and prayers during this extremely difficult time," Brewer said. "We have deployed a massive amount of resources and teams. We have basically tripled the number of assets in the area."
The missing children were identified as 2-year-old Matilda "Mattie" Sheils and 9-month-old Conrad Sheils, according to the Upper Makefield Police Department. The children's 32-year-old mother, whose body was recovered on Sunday, was identified as Katie Seley, police said.
The missing children's father and Saley's fiancée, Jim Sheils, released a statement through police saying he and his family "would like to thank everyone for their prayers and support. They would also like to thank all of the personnel involved in this massive search effort to find Mattie and Conrad."
The family is from Charlestown, South Carolina, and were visiting relatives in Bucks County when tragedy struck on Saturday, police said. The family was driving near Hough's Creek, a 5-mile tributary leading to the Delaware River, when torrential rains struck at about 5:30 p.m., causing the creek to overflow and trapping the family in their car on Route 532, authorities said.
Jim Sheils and the children's grandmother grabbed ahold of the missing siblings' 4-year-old brother and escaped their car as it and other vehicles were being washed away, according to officials. Seley grabbed her two younger children and exited the car, but they were washed away in the powerful flash flood, officials said.
The father, grandmother and 4-year-old were found alive, officials said.
Brewer said Monday afternoon that he wanted to make it clear the family did not get trapped in their car by driving into high water.
"They were caught in a flash flood," Brewer said. "The wall of water came to them, not the other way around."
Besides Katie Seley, four other people were confirmed dead in the flooding. They were identified by the Bucks County Coroner's Office as Enzo Dipero, 78, and Linda Dipero, 74, both of Newtown Township, Pennsylvania; Susan Barnhardt, 53, of Titusville, New Jersey; and Uko Love, 64, of Newtown Township.
The coroner's office said all of the victims died from drowning.
Brewer said 11 vehicles, including the South Carolina family's car, were found stuck in the rising flood waters near the Washington Crossing area, where George Washington made his famous crossing of the Delaware River in December 1776 during the Revolutionary War.
Brewer said search-and-rescue crews witnessed at least three vehicles being swept into the creek, including one that was found 1.5 miles away from where it went into the creek.
At least 75 search-and-rescue crew members were combing the area for the missing children, Brewer said. He said the number of search-and-rescue personnel is expected to grow to over 100 on Monday as more equipment is being brought in to aid in the search.
He said K-9 units, divers and aircraft are being deployed in the search, as well as drones.
"We're in the area of the creek and widening the search," Brewer said Monday. "This is going to be a massive undertaking. We are searching the entire creek."
Meanwhile, New Jersey Gov. Phil Murphy declared a state of emergency for parts of sections of Warren County near the Delaware River that were also devastated over the weekend by flooding. On Monday, Murphy toured the hard-hit town of Belvidere and said it was a "small miracle" there were no injuries.
Officials said the heavy rain on Sunday toppled trees, caused landslides and washed away roads in Warren County, including Route 46 in Knowlton Township.
"We're going to stay here as long as it takes," Murphy said of the cleanup underway. "Incredibly important for people watching this to document every penny you spend."
At least $50 million in storm damage over the past week occurred in Suffolk County, New York, on Long Island, where 5 inches of rain fell in two hours on Saturday, overwhelming storm drains and damaging roads.
ABC News' Darren Reynolds contributed to this report.