It started with small flames inside garbage bins. Then over the summer and early fall, fires set in Andover parks ballooned in number and intensity, with the most recent blaze burning an entire playground set and causing more than $30,000 in damage.

Park officials say it’s the worst park vandalism in recent memory, with at least six fires scorching structures at various city parks since June. An arson tip line set up this month has helped investigators narrow their search for suspects — a notoriously tricky task given how little evidence a fire leaves.

“Forensically, it’s very difficult to find an arson suspect,” said Cmdr. Paul Sommer, a spokesman for the Anoka County Sheriff’s Office. “What’s left behind is burned up.”

The state chapter of the International Association of Arson Investigators is offering a reward of up to $5,000 for tips that result in an arrest.

Most of the tips funneled to the Sheriff’s Office so far have pointed to teenagers, Sommer said, adding that investigators are building a case from several leads.

The blazes began in late June with a string of recycling bin and dumpster fires at Purple Park, as well as damage to a pavilion, said Andover Fire Chief Jerry Streich.

Fires soon popped up in Timber Trails Park and Sunshine Park, with more dumpsters, garbage bins and also portable toilets going up in flames. At Sunshine Park, two pavilions were also defaced with spray paint.

The biggest blaze burned the night of Sept. 29, when a fire consumed a playground set in Shadowbrook East Park, resulting in $38,000 in damage. Flames licked up the structure’s steps and scorched or melted much of its plastic surface, including all of the rock climbing wall.

Citywide, the fires have caused about $50,000 in damage, Streich said.

“We can’t tolerate this,” Streich said. “Fire can be used just like any other weapon to create fear, and we have to stop it.”

Since the fires began, the city has cleared out brush and foliage to make once-secluded spaces in parks more visible. Extra patrols have been set up near various parks. No fires have been reported since the playground blaze last month.

At Shadowbrook East Park, city workers dismantled and removed the scorched playground set earlier this month and don’t expect the replacement structure to be up until next spring, said Todd Haas, Andover’s assistant public works director and parks coordinator.

In his 30 years with the parks department, never has so much vandalism struck spaces meant for recreation and play, Haas said.

“It’s too bad,” Haas said. “It impacts kids the most.”