Razor blades have been discovered in Bonita Cove Park at least four times over the past 18 months.
The first of Sunday's blades were found around 12:15 p.m. in the park, next to West Mission Bay Drive at Mission Boulevard, according to San Diego police.
Police checked playground equipment and carefully searched through sand and grass with metal detectors.
“That’s really evil – I don’t know who would do such a thing," said Shakeh Boghoskhanian, a mother who took her daughter to play at the park.
No injuries were reported, San Diego police Officer Dan Lasher said.
“[The razors] appear to have been placed in the area within the last 24 hours,” Lasher said. “Hopefully we can put a stop to it soon.”
At least three razor blades were found stuck into grass with sharp edges facing up on Sept. 13, but no one was injured in that incident.
In late April, 19 blades were found in the park's grass. A 7-year-old boy and a 15-year-old boy cut their feet on razor blades, which police said then were intentionally planted.
And in August 2013, a 41-year-old woman was cut by one of a half-dozen blades left there.
The park is heavily used by children, as it is across the street from Belmont Park and next to a popular Mission Bay swimming area.