UPDATE: The search has resumed today and includes three ground teams plus a helicopter, drone and a motorized boat, which was checking along the banks of the Colorado River near its confluence with the creek, park spokeswoman Robin Martin said.
The ground teams, a total of approximately 20 people, remained in the canyon overnight, Martin said.
ORIGINAL STORY: PHOENIX - Authorities were searching Grand Canyon National Park on Monday for an eighth-grade McCallie School student and his stepgrandmother, who were swept downstream Saturday as they tried to cross a creek over the weekend in a remote part of the park.
Two fellow hikers in their group alerted officials by setting off an emergency GPS locator beacon in the backcountry below the canyon's North Rim, according to Chief Ranger Matt Vandzura of the National Park Service.
This undated photo provided by Mark McOmie shows Jackson Standefer. (Mark McOmie via AP)
This Sunday, April 16, 2017, photo provided by the National Park Service shows Tapeats Creek in Grand Canyon National Park in Arizona. Authorities are searching for Jackson Standefer, 14, and Lou-Ann Merrell, 62, after the pair lost their footing Saturday and fell into the water during a family trip in a remote area of the Arizona park. (National Park Service via AP)
This undated photo provided by Mark McOmie shows Jackson Standefer. Authorities searched Grand Canyon National Park on Monday, April 17, 2017, for eighth-grader Standefer and his step-grandmother who were swept away as they tried to cross a creek during a family trip in a remote part of the park. Chief Ranger Matt Vandzura says the two lost their footing Saturday. (Mark McOmie via AP)He said the boy, Jackson Standefer, 14, and his 62-year-old stepgrandmother lost their footing and were swept away in Tapeats Creek, a tributary of the Colorado River that runs through the Arizona landmark.
Mark McOmie, the uncle of the missing boy, told The Associated Press the teen, his mother and the stepgrandparents were crossing a water trail Saturday evening when the boy and his stepgrandmother fell in and were swept away by the water.
McOmie identified the stepgrandparents as Randy Merrell, who in 1981 help found the Merrell Boot Co., and his wife Lou-Ann. He said both are avid hikers and knew the area well.
A call to the boot company's Indiana headquarters wasn't immediately returned Monday.
Lou-Ann Merrell is "a very experienced backpacker," McOmie said. "If they can get to a spot where they cannot be in the water and stay warm, she's got the skills needed to get them through it. The odds aren't great. But given their skills and knowledge of the area, that will probably lead to the best possible outcome."
McOmie said search and rescue teams had found both their backpacks and belongings inside it, but the family has interpreted that with mixed feelings.
On the one hand, he said, it looks as if they were able to get their backpacks off. "The bad part is that they don't have their gear," McOmie said.
Vandzura said it's too early in the investigation to determine what went wrong.
In a statement to WRCB on Monday, Standefer's family said: "The outlook is not promising, but there is still a glimmer of hope. We will hold on to this hope until there is reason not to. Please keep praying."
A statement from the school said the family "remains hopeful that rescuers will find Jackson and his step-grandmother and return them safely to their families." It noted students were told of the situation and rescue operation and offered help if they desired it.
"The entire McCallie community sends its prayers to the Standefer family and all those close to Jackson as we all hope for a positive ending to this situation," it read.
No rain or flash flooding was reported in the area, but it was not known whether the water level of the creek was higher than usual.
Creeks in Grand Canyon often see higher water levels in the spring as snow melts, Vandzura said.
The Park Service describes conditions in the area on its website, warning that melting snow or heavy rain may make crossing the creek impossible. It also said hikers can use a "sketchy, seldom used trail" that lets them walk around the creek when the water is high, but that the path should be used only as a last resort.
Vandzura said the four hikers in the group were on a different trail that crosses the creek.
An Arizona Department of Public Safety helicopter searched for the missing pair Saturday night, and the Park Service sent a helicopter and several ground crews to comb the area Sunday.
The Park Service resumed the search Monday with ground crews, a helicopter and a drone.
McCallie is an all-boys boarding and day school founded in Chattanooga in 1905. It has 931 students in grades 6-12 from 22 states and 10 countries.