Pet owners are being warned to keep their animals out of Lady Bird Lake after the death of two dogs that swam there, Austin city officials said in a statement Monday.

Although the city has yet to confirm the causes of the deaths, a warning was first issued Sunday night "out of an abundance of caution" because the dogs' symptoms appeared consistent with exposure to a toxin produced by a type of algae, city officials said.

Watershed Protection officials were observing the lake during water-quality testing and noticed something that looked like blue-green algae, said Bryce Bencivengo, spokesman for the city's Homeland Security and Emergency Management.

"Preliminary results indicate the algae is a type of blue-green algae of the genus Oscillatoria," the city said Monday. "This type of algae can release a neurotoxin ... (that) can be harmful to pets and people if a sufficient quantity of water or algae is ingested."

RELATED: What you need to know about the toxic ‘algae’ growing in Austin’s Lady Bird Lake

City officials said the algae is especially prevalent near Red Bud Isle near Tom Miller Dam and was covering up to 40% of the water's surface there.

"The situation is evolving. We have not seen any impacts to aquatic species at this time," the city's statement said.

According to the city, the algae bloom "appears to be confined to algae growing on the bottom of the lake and then floating in clumps to the surface." Results from the analysis for the presence of the toxin should be available early next week, the city said.

Swimming in Lady Bird Lake is already illegal, but the risk to humans, such as through ingestion of lake water, cannot be known until test results are analyzed, the city said. In the meantime, officials said, boating remains a safer option.

City officials also reminded residents that Austin Water does not use Lady Bird Lake as a drinking water source, and the algae is not affecting the city’s water system.

According to the Merck Veterinary Manual online, blue-green algae can produce toxins that are poisonous to dogs. Some symptoms of algae poisoning include vomiting, diarrhea, jaundice, loss of appetite, stumbling, seizures and excessive salivation.

Earlier Monday, Flapjack, a black Labrador-mix sporting a dark blue collar, chased a red ball as he ran in and out of Lady Bird Lake at Vic Mathias Shores near the South First Street bridge.

The 2½-year-old dog was with Corey Hilton, 30, who was pet-sitting for a friend. Hilton had no idea that Austin city officials on Sunday night began warning pet owners to keep their animals out of the lake because of algae that could be dangerous.

"I haven't heard anybody really talking about it," Hilton said of the warning. "It was a little weird to me that I noticed nobody was down here today. I thought there'd be a couple people."

Hilton said it would be helpful if the city put out signs at the lake to warn people who maybe don't keep up with pet news.