CHICAGO (CBS) — Large crowds on the lakefront are again drawing the ire of Mayor Lori Lightfoot, who warned Saturday evening on Twitter that she might be forced to close the city’s lakefront again if people don’t stop what she called “reckless behavior.”

The number of new COVID-19 cases, and the positive test rate for the virus have been climbing steadily in Chicago and across Illinois for the past several weeks, prompting Lightfoot and Gov. JB Pritzker to caution that they might be forced to resume some restrictions that were lifted earlier this summer if new cases aren’t brought under better control.

Lightfoot sounded the alarm again on Saturday, sharing a photo on Twitter of a large crowd gathered at Montrose Beach, with few if any masks in sight, and most people in the picture not keeping six feet away from each other, the two most basic public health measures recommended by the CDC and local health departments to slow the spread of the coronavirus.

“It’s called a pandemic, people. This reckless behavior on Montrose Beach is what will cause us to shut down the parks and lakefront. Don’t make us take steps backwards,” Lightfoot wrote.

The mayor later posted that she personally visited the beach and said “it’s being addressed.”

Several police vehicles were spotted at Montrose Beach not long after the mayor’s tweet, and several smaller groups of people were seen leaving the area.

Lightfoot shut down the city’s lakefront trail, lakefront parks, and beaches in late March, along with the 606 trail and the Riverwalk, after too many people ignored her warnings to avoid large gatherings as the pandemic was in its infancy and just beginning to spike in the first wave in Chicago.

The Riverwalk reopened in mid-June, and the lakefront and 606 trail reopened about two weeks later, although the beaches remain closed, and visitors are required to “keep it moving” on the lakefront.

Since the city began its gradual reopening process earlier this year, Lightfoot has said many times she does not want to force businesses to shut down again, or close public spaces again, but she said she won’t hesitate to do so if people don’t observe proper public health precautions to prevent the spread of the virus — particularly by wearing masks in public, and staying six feet away from others whenever possible.