The police department that oversaw protests in Ferguson over the death of Michael Brown attracted new criticism on Thursday after weighing in clumsily on the fatal police shooting of Tamir Rice, a 12-year-old boy in Ohio who was playing with a replica gun.

In a Facebook post headed “Kids will be Kids?”, St Louis County police told parents to warn their children that if they prompted an emergency call by playing with toy guns in public, “police will respond as though it is a real gun”. The post was linked to from the department’s Twitter account.

“Pellet guns and Airsoft guns should not be allowed to be played with throughout the neighbourhood, common grounds, or used to threaten or intimidate people,” said the post, which was written by officer Aaron Dilks from the City of Fenton precinct.

The post met with widespread dismay on social media. “Insanity,” wrote Shaun King, an activist and writer. “Stranger than fiction that they’d do this.”

Tamir, who was pointing his BB gun while wandering alone around a park in Cleveland, was immediately shot dead by an officer responding to a 911 call. The caller told the dispatcher: “I don’t know if it’s real or not.” This was not conveyed to the officer. Tamir’s $20 pistol lacked an orange tip that typically distinguishes toy or pellet guns from full-powered versions.

St Louis County police’s tweet. Photograph: Screen capture

The 911 caller also suggested that Tamir was repeatedly pulling the BB gun out of the waistband on his pants. The St Louis County Facebook post said on Monday: “Remember, if an Airsoft gun is tucked in your pants like a holster then obviously the orange tip is no longer visible.”

The St Louis County message also offered parents “tips to help your child respond appropriately” if they were confronted by police while holding a toy or pellet gun, such as “do not run away” and throwing the toy gun away from their hands. “They may be ordered to lie down on the ground,” it said.

The Facebook post, and a tweet that it automatically triggered on the official St Louis County police Twitter account, were both removed later on Monday morning. Asked during a brief phone conversation whether he wrote the post, Dilks told the Guardian: “Yeah, it was me.”

Dilks prefaced his post by stating that he was not making a judgment on whether the shooting was justified. “This is about the Fenton Precinct making residents aware of a ‘hot’ topic and learning from this incident so Fenton never loses a child’s life,” he wrote.

After confirming that he wrote the post, Dilks referred all other inquiries to a county force spokesman. When asked whether he regretted the post, Dilks hung up the phone. The county force was expected to issue a statement later on Monday.

The St Louis County police is under a joint review of practices with the US department of justice following the controversial policing of protests following Brown’s death in August. The force’s military-style response was heavily criticised.

The department became more active on social media earlier this month as protests mounted in advance of a grand jury deciding not to indict Darren Wilson, the Ferguson police officer who shot Brown dead in August this year. Recaps of protests were posted to Facebook and some were even live-tweeted.

Asked by the Guardian earlier in November why the department was tweeting more often, a spokesman said: “Social media is a new way for our department to attempt to get information out as quickly as possible. That is why we are doing it. It is done by a member of the public information staff.”

Jon Belmar, the St Louis County police chief, later issued a statement apologising to Tamir’s family over the Facebook post, which he described as “misguided” and “offensive to many people”.

“I realize the message was insensitive to Tamir’s family and the sorrow they are currently experiencing,” said Belmar. “Our continued thoughts and prayers go out to Tamir’s family in this trying time.”

Belmar said he was not aware of the post before it was published and added that the department’s social media policies had been altered as a result of the furore.