Melted plastic sagged from a large plastic tunnel and a stench hung in the air at the Lyndale Community School playground, where about half the play area was roped off with yellow caution tape Monday.

The playground was set on fire Sunday, the fifth time a public school playground in Minneapolis has been targeted by arson since mid-June.

The string of fires has frustrated parents, puzzled district officials and stymied authorities.

No one has been arrested. Neither police nor fire officials will say the fires are connected.

"Who could have done that? It's unbelievable," parent Tenzin Nyima said after one of his two children attending Lyndale pointed out the damage Monday. "Schools don't have money to spend on stuff that's destroyed," added Carrie Swanson, who has four students at the school.

The blaze created heavy smoke and fire about 2 p.m. Sunday outside the school, at 312 W. 34th St., before being quickly extinguished, authorities said. Police say they are treating the fire as an arson.

All of the targeted playgrounds remain unusable, said district spokeswoman Larissa Chinwah, who said total damage has swelled to about $220,000.

It was the second case of an intentional fire at Lyndale; a slide was torched four years ago, according to parents.

Mark Hinds, executive director of the Lyndale Neighborhood Association, which years ago paid for playground equipment at the school and at a nearby park, called the fires unusual. "We just haven't seen a lot of arson," he said.

Parents and the community raised some $7,000 after the earlier fire to replace the damaged equipment and to add some more. Chinwah said the district is still trying to determine how it will replace the equipment in the wake of Sunday's fire. The district is generally self-insured, but damages over a certain level are covered by an outside insurer.

"We're really frustrated," said Jason Matlock, the district's director of emergency management, safety and security. He said some schools have video cameras, but even with those it can be hard to determine when horseplay turns to vandalism. He called for community members to help keep an eye on playgrounds.

"We're not going to go to a place where we close everything off," Matlock said.

The fourth fire occurred on the afternoon of the previous Sunday, Sept. 1, at Jordan Park School (Hmong International Academy and Pierre Bottineau French Immersion). That torching created heavy black smoke and flames from the rubber mulch on the ground, fire officials said. The smoke set off the school alarm.

The first and most extensive blaze occurred June 13 at Lucy Craft Laney at Cleveland Park Community School. Along with the playground, exterior masonry, windows and doors were damaged.

Next came two fires set at the Mona Moede early childhood and pre-K school on Aug. 5 and Aug. 21.

Anyone with information about any of the fires is asked to call the Minneapolis Fire Department's arson hot line at 612-673-3070 or 1-800-723-2020.

A reward of up to $2,500 is being offered.