WASHINGTON (AP) — U.S. health authorities are alerting consumers to a new scam involving fake government warning letters sent to people who tried to buy medicines online or over the phone.

The Food and Drug Administration said Friday that the fake letters may be part of an extortion scam. However, authorities have not yet documented cases of consumers being coerced to turn over money.

The forged letters claim to be from the FDA or the Federal Trade Commission, but those agencies almost never issue such warnings to private individuals, but rather to companies, professionals or industry officials. The letters falsely claim that the government is investigating the drugs the consumers attempted to purchase.

FDA officials have repeatedly warned about the risks of buying medicines through unverified online pharmacies.