(CNN) Three siblings died and one survived after the four were overcome by waves while swimming in Texas, authorities said.

The four children between the ages of 7 and 13 years old were swimming at Sylvan Beach Park in La Porte on Thursday night, according to Harris County Sheriff Ed Gonzalez. The family has not yet been identified, but Gonzalez said he understood that they are recent "transplants to the Houston area from out of state," he told CNN affiliate OnScene.TV.

Authorities recovered the bodies of two siblings from the water and continued to search for the body of the youngest sibling, a 7-year-old boy, Gonzalez tweeted . An hour later, he tweeted that the body of the 7-year-old had been found.

The other siblings that drowned were an 11-year-old boy and a 13-year-old girl, according to a statement from Chambers County Sheriff Brian Hawthorne

A fourth sibling, a 10-year-old boy, survived. A bystander, 23-year-old Veronika Alvarez, heard her dog barking at the situation, saw the parents trying to grab a child, jumped into the water and saved the boy, she told CNN affiliate KTRK/ABC 13

"I just jumped in because no one was responding. Everybody was quick to grab their phones, but I was like, it's not important. What's important is the people who are drowning," Alvarez told KTRK

She could see the drowning boy was "in shock, he wasn't swimming, I saw that he was going down," she said, adding she kept telling herself to "keep him above water, keep him above water."

Alvarez said she was taken to a hospital, where she was treated for having inhaled "small amounts of water" into her lungs. The boy was treated by EMS at the scene and released.

Sylvan Beach Park is a county park in La Porte, a city located east of Houston on Galveston Bay.

Police received a call to report the drowning around 7:15 p.m. and arrived on the scene within minutes, Gonzalez told OnScene.TV. Dozens of emergency responders, including local police and fire departments and units from the Coast Guard and Port of Houston, were on the scene with drones, helicopters, trucks and ambulances. Beachgoers, some still wearing swimsuits and draped in towels, watched the sad effort from the shore.

"Things happened really quickly," Gonzalez said. "A lot of people were out here, the kids were playing, mom and dad were here as well." He added that the parents "are very distraught. There's no way to make it any softer."

The total search time for all three siblings was about four hours, Gonzalez told OnScene.TV.

The reason the children drowned is still unclear. They may have been swimming beyond a water safety buoy and encountered strong tides and rip currents, Gonzalez said in a tweet . Later, he told OnScene.TV that police are looking into whether a large boat went by, creating rough waters.

Alexis Lawlis, of Houston, did not witness the tragedy herself, but she had spent hours on the beach with her five children, who range in age from 9 months to 15 years old. She and her family watched the emergency response into the night.

"It's devastating," she told OnScene.TV, adding that she felt "heartbroken."

Alvarez told KTRK that when she jumped into the water, she thought there was only one drowning child and only later learned that three others had not been saved.

"I just started crying," she said. "I want to attend the funeral. I want to let the little boy know it's okay ... it's not his little fault."