ST. LOUIS COUNTY, Mo. – Police in St. Louis County are warning people about using “buy and sell” smartphone apps after three robberies and one attempted robbery during would-be purchases over the weekend. One of the victims was shot, police said.

The victims were using the Letgo and Facebook Marketplace apps for deals on iPhone Xs and an Xbox game console in one instance, police said.

According to police, the victims agreed to meet the supposed seller in seemingly safe areas in north county, just off of South Florissant road near a school in Cool Valley around 5:30 p.m. Sunday. The other three incidents were along Bermuda Road near Bermuda Elementary School. One of the robberies “went bad” and the victim was shot in the leg.

All the crimes were during daylight hours with the first case late Saturday afternoon and three more Sunday. Investigators fear there may have been more cases.

“In a matter of 36 hours,” said Sgt. Tameika Sanders, spokeswoman for the Normandy Police Department. “This all developed very quickly. While we believe a lot of these incidents are related, it’s one of those unfortunate things where there’s these unsuspecting buyers that see this deal that just appears too good be true and it actually was, police departments would rather have the exchanges taking place at their departments than to take robbery reports and deal with citizens being injured and citizens that could potentially be killed.”

Buyers should never go to a meeting alone, never carry more money than needed for the transaction, and insist on making purchases at the nearest police station, she said.

If the seller does not want to meet at a police station, it’s a sure sign of trouble.

Police said they have strong leads and would not release any suspect information in these cases.

You can read a statement from Letgo below, posted in its entirety:

Our team’s been in contact with local police and we’ve offered our assistance with their investigation.

Nothing will ever be more important to us than the trust and safety of our users. That’s why letgo includes things like user profiles, ratings and reviews, which are specifically designed to let you see who you are chatting with and what others have said about them. We use a combination of human and artificial intelligence to help keep letgo users safe and we ask our user community to help us by flagging content or other users that raise concerns. When they do, we quickly take the appropriate action, which may include permanently blocking a user in extremely rare cases where it’s necessary.