AUSTIN, Texas -- The City of Austin is still investigating blue-green algae that popped up at Lady Bird Lake and Redbud Isle.

• Pet owners warned to keep animals out of Lady Bird Lake, Redbud Isle
• Pet owner Claire Saccardi's dog died following swim 
• Test results expected Friday 

City leaders announced Wednesday Redbud Isle would be closed until further notice.

One pet owner reported her dog died after swimming in the park on Thursday. Claire Saccardi said it was the first time the two had been to the park.

"She was swimming in the water, going back and forth, playing with other dogs. She was running around. She kept running back to me with the biggest smile on her face like saying, 'Thank you, Mom, for playing with me today,'" Saccardi said. 

After about an hour, she came home to cook dinner and give her dog, Harper, a bath.

"When I look back on it, it just feels like a horror movie because it was so happy and so perfect, but yet this thing was looming over us this whole time," she said.

Saccardi realized something was wrong when Harper tripped and fell.

"She gives me this really sad look, just like very confused like 'What's happening to me mom?'" she said, emotionally.

Harper couldn't use her front legs, so a neighbor helped carry her to the car to go to an emergency vet. Minutes later, she passed.

After notifying the city, blue-green algae was spotted at Red Bud Isle and Lady Bird Lake. The algae can release a toxin and can cause respiratory paralysis and death in pets. Tests are being done to determine if toxins are present in the lake. A preliminary report should be available Friday.

RELATED: City Urges Pet Owners to Keep Animals Out of Lady Bird Lake 

City leaders said the algae forms in the heat and slow water flow. Officials believe zebra mussels could also play a role in the new algae blooms.

Pet owners are being warned to keep their dogs out of the water. There have been no known reports of humans getting sick from the water.

Saccardi says this tragedy has been a nightmare for her, but she takes comfort in sharing her story and possibly helping other pet owners.

"More than anything, I just want people to know about this because it was something that I had never heard of. Of course, I would have never gone if I had heard of it," she said.

The city says they test the water routinely for algae.