LITTLETON, Colo. -- The swimming beach at Chatfield State Park was temporarily closed Sunday when routine water testing found high levels of E. coli bacteria, authorities said.

But 7NEWS Reporter Jaclyn Allen found dozens of people playing on the beach and swimming in the water right next to the closed swimming area Sunday afternoon.

Several families said they had no idea why the beach was closed because it's not posted on the signs, which simply said: "Swim Beach Closed Today." The swim beach was also fenced off.

Visitors were surprised to learn from 7NEWS that the closure was triggered by bacterial contamination that could make them seriously ill.

"I had no idea about the E. coli," said Monique Garcia, whose toddler son had been playing in the water all day. "The woman who took our money told us the beach was closed because the water quality was not good."

Signs warning "Swimming Prohibited" were posted in the neighboring area where people were swimming and splashing in the water. Park rangers said people are only allowed to wade knee-deep in the water outside the designated swimming beach area.

But clearly people weren't getting the message.

Later in the afternoon, park rangers came to warn people it wasn't safe to go underwater -- because of boats in the area.

Allen did find small, 8-by-11-inch signs at the park entrance post, warning: "Swim Area Closed Due To High Ecoli Counts."

Park officials said bacterial contamination in shoreline water in unusual.

"Closures due to high bacteria counts occur very rarely at Chatfield and I am only aware of four other closures since the 2000 swim beach season," said Keith Kahler, manager of the Littleton area park.

Health officials indicated that the E. coli contamination may have been caused by recent heavy rains and high temperatures, Kahler said. The regular water quality tests, performed to meet Colorado Department Public Health and Environment requirements, contained higher than allowable bacteria levels.

Park officials are awaiting new test results Monday. If the water quality is safe, the beach will reopen Tuesday.