Brentwood’s Roberto Clemente Park — shuttered for more than three years after the discovery of contaminated materials dumped there — reopened Monday morning, just in time for sisters Karen and Brittany Robles to say goodbye.

The sisters, who live near the park and grew up playing there, were moving to Mastic later Monday.

“We would spend our summers here on the swing sets,” Karen Robles, 24, a real estate agent, said as she took one last turn riding on the swings.

The park had been closed since April 2014, when county officials discovered that more than 40,000 tons of contaminated construction debris had been illegally dumped on the park’s soccer fields and in a drainage basin.

The Suffolk County district attorney’s office began an investigation, which expanded to three other sites in and around Islip Town, after the state Department of Environmental Conservation received complaints about illegal dumping at the park of substances including glass, concrete, cement and other contaminated materials and tests showed varying levels of harmful toxins.

The investigation led to the convictions of five men, including two former town employees.

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While town officials had promised the park would be reopen by the start of summer, that date came and went, leaving residents to express frustration that the community has gone without its largest public park for years.

Town spokeswoman Caroline Smith said the town had planned to open the park sometime this week, but after favorable weather conditions allowed park workers to finish laying topsoil, the town decided to open the park Monday. “It is officially open to the public for fun,” she said.

Bids to rehabilitate the park’s Olympic-sized pool are due Thursday, with the pool and a new spray park currently slated for completion in summer 2018.

Robles said she thought the town took “longer than expected” to reopen the park, but the finished work “looks amazing.”

But community activist Nelsena Day, who had grilled the Islip Town board over the plans for the park’s reopening at many board meetings, gazed around the playground with a critical eye Monday.

“We fought long and hard” for this reopening, Day said, noting that the parking lots still had potholes and uneven curbs that she wants fixed. “I am more concerned about the safety of the children,” she added.

The park’s official reopening shortly before 8 a.m. Monday drew a smattering of local residents, including Lesly Gutirrez, 11, and her 5-year-old brother, Brandon, who were the first kids to hit the playground set. They biked over with their mother from home a couple blocks away and tumbled down the slides on the jungle gym.

“She’s happy because she can bring us to play,” Lesly said, translating for her mother.

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Ramell Mazyck, 22, a mechanic, shot solitary hoops on the newly resurfaced basketball court. “I was here on the last day the park was open, when they told us to leave because they were going to rebuild it,” he said. “It’s funny that I’m here on the first day it’s open, too.”

Marianne Gonzalez brought her grandchildren Victoria, 6, and Charlie, 4, to check out the playground at the park on Monday.

“It’s safe, right? I hope,” Gonzalez said, looking around the sand-filled area around the swing set. “I’m just so glad they reopened. It looks nice.”