The children are members of a South Carolina family that was visiting relatives when they got caught in a flash flood Saturday that claimed the lives of five people, including the mother of the missing children.

WASHINGTON CROSSING — Crews in Bucks County on Monday intensified the search for a missing 9-month-old boy and his 2-year-old sister, swept away after weekend rains swelled the banks of a creek while they were driving to a barbecue with their family.

Authorities described Monday’s search for missing Matilda Sheils and her brother Conrad Sheils as a “massive undertaking” including 100 search crew members and numerous drones dispatched along a creek that drains into the Delaware River. The children are members of a Charleston, South Carolina, family that was visiting relatives and friends when they got caught in a flash flood Saturday.

“As they tried to escape the fierce floodwaters, dad took his 4-year-old son while the mother and the grandmother grabbed the two additional children,” said Upper Makefield Township Fire Chief Tim Brewer. While father and son made it to safety, “the grandmother, the mother, and the two children were swept away by the floodwaters,” he said.

The children’s mother, Katie Seley, 32, was among at least five people killed in the flooding, while the grandmother survived, Upper Makefield Police said in a social media post. The other victims’ names have not been released.

Monsignor Michael Picard of St. Andrew Roman Catholic Church, where family members are parishioners, said he spoke with the grandparents Sunday. The grandfather attended morning Mass on Monday, where the family was included in prayers.

“No matter how long I’ve been doing this — over and over and over, many, many years — you find yourself still helpless and without words to make people feel more comfortable,” Picard said. “And so you just simply pray with them for a few minutes.”

Colleen Courtney, attending a church conference near the search scene Monday, was among those praying for the families.

“It’s just such a tragedy and just so much grief, I’m sure, and mourning that’s going on right now. I pray to find these children,” said Courtney, of Ewing, New Jersey.

Check back for updates on this developing story.

Keystone senior community editor Patrick Berkery contributed to this report.