ZAGREB, Croatia (AP) — Traces of a cleaning agent have been found in the fizzy drink that was served to a man in Croatia who was later hospitalized with a throat injury, triggering a nationwide alarm, police said Thursday.

An analysis of the bottle and glass served to the man at a cafe last weekend in the northern port of Rijeka showed that the contents of both included a highly alkaline substance. But it remains unclear how it got there, and other similar drinks that were tested were problem-free, police said in a statement.

The incident triggered alarm throughout Croatia, with dozens of people complaining of similar problems in recent days, but authorities have said no serious injuries have been detected.

“Based on the established chemical composition, it can be concluded that it was some kind of a washing or degreasing agent,” police said.

Police said other drinks, including 20 unopened bottles of the same drink that the man had, have been tested and found to have no problems.

Police did not identify the drink, but previous reports said that the man hospitalized in Rijeka over the weekend with an esophagus injury had been drinking Romerquelle Emotion Blueberry Pomegranate, a Coca-Cola brand, from a glass bottle.

Coca-Cola in Croatia had temporarily pulled a number of its drinks from shelves, and said it was cooperating with the investigation. It also said that its own preliminary review had found nothing out of the ordinary.

Croatia’s Health Minister Vili Beros sought to play down any concerns of a widespread problem.

“It was an isolated case,” Beros said. “A criminal investigation that is underway will determine all the circumstances that led to this event.”

The substance was “either planted or was an unintentional mistake,” said Beros. ”It is hard to say how it happened.”

Out of 45 people who have complained of similar issues since the incident, only four of them have shown any substantial effects on their digestive tracts, Beros said.

“There is no reason to panic,” Beros said, adding that authorities will continue to monitor the situation.

Coca-Cola in Croatia said its “key priority” was supporting the authorities in their investigation.

“We fully understand public alarm caused by speculation and false information in the past several days,” the company said in a statement carried by the state HRT television.