Ocean City police were investigating a possible drowning Sunday, the morning after a child disappeared beneath the waves on one of the busiest beach weekends of the summer.

The 7-year-old girl was playing in waist-deep water at the beach in Ocean City, Md. on Saturday evening, before getting knocked down by a wave and disappearing beneath the surf, a spokeswoman for the Ocean City police department said.

Rescue divers from the local fire department responded to the scene and were able to locate the girl, but she was pronounced dead a short time later at Atlantic General Hospital, said Jessica Waters, the spokeswoman.

The incident occurred sometime after 6 p.m. when lifeguards were not on duty.

“The most important thing people need to know is that the ocean is unpredictable. It can be rather calm and rip currents can form rather quickly. Even the most experienced swimmer should not be swimming in the ocean when the lifeguards are off duty,” Waters said.

Officials have not released the girl’s name.

Ocean City, a popular Maryland beach resort town, can draw between 200,000 and 250,000 visitors on a nice weekend like this one, Waters said.

But the ocean has been choppy in recent days. Last weekend rescue personnel and lifeguards carried out “quite a few rescues” along the resort town’s long strip of beach, as well as a few more on Sunday morning, Waters said.

Waters said that police believed drowning to be the likely cause of the young girl’s death Saturday night, but that police investigators were awaiting an autopsy and interviewing witnesses Sunday. They believe that she was in swimming in the company of friends, rather than her family, at the time of the incident.