Fire Chief Ronald Blackwell said the injured people, including a 3-year-old, were hospitalized with non-life-threatening injuries.
Authorities had feared others were trapped inside the 45,000-square-foot building, but rescuers found no one in an initial search and Prince George's County Police reported no missing-person calls, officials said.
Dogs and thermal imaging were used in the search.
Firefighters were planning another search, but Prince George's County Executive Jack Johnson said he remained "cautiously optimistic" that everyone in the store was able to escape.
In Virginia, the roof of an elementary school in Herndon also collapsed during Saturday's rain, Fairfax County officials said. The school was empty at the time.
A Wal-Mart store in Leesburg, about 20 miles west of the Toys 'R' Us, was evacuated as a precaution after the roof sagged, said Mary McGuire, spokeswoman for Loudon County fire and rescue.
About 90 percent of the Toys 'R' Us roof fell in, all the way from the cash registers to the back of the store, Prince George's County Fire Department spokesman Mark Brady said.
"We saw beams falling and we heard people screaming," one shopper, a grandmother, said to CBS News, Radio.
"We thought we were gonna die. It was chaotic," another shopper told CBS News, Radio.
"It looks like a bomb went off inside," said Ben Messier, general manager of a Dodge dealership across the street, on CBS News, Radio. "There's just debris everywhere, stacked up 12-15 feet high."
Doretea Burton was with her mother and two children, a 1-year-old boy and 7-year-old girl, shopping for a high chair when she heard "a roaring sound."
"You could see the sky beams falling and beams bent," said Burton, 29. "For a moment we all thought we were going to die."
All 10 employees in the building at the time of the 11:45 a.m. collapse were unharmed. Store manager Mike Molnar said he didn't know how many customers were in the building, but Burton estimated there were about 50.
Burton said she heard screams and saw one woman with a bloodied head. She said she and her family had to walk circles around the rubble to get out.
"The roof was coming down as we ran. It was like it was following us," she said.
Brady said the combination of the flat roof on the free-standing toy store and recent snow and rain created a "disaster waiting to happen."
The accident was the worst of several roof collapses in Maryland and Virginia, which also saw some flooding after heavy rain followed snow earlier in the week that reached more than 2 feet in some places.
The heavy rain added to the weight of the already heavy snowpack on many buildings, said Don Lumpkins, operations officer at the Maryland Emergency Management Agency.
State emergency officials received scattered reports of collapses of retail buildings, barns and garages throughout the area, Lumpkins said. Only the Toys R' Us accident caused possible human injuries, he said.
On Monday, a record 28.2 inches fell at nearby Baltimore-Washington National Airport and more than 16 fell at Washington's Reagan National Airport.