WASHINGTON — A man who was unhappy with the treatment he had received at a Minnesota health clinic opened fire inside the facility Tuesday and wounded at least five people, authorities said.

Police arrested the suspected shooter, identified as Gregory Ulrich, 67, who was known to law enforcement from previous encounters.

He allegedly went into the Allina Health Clinic in Buffalo, Minnesota just before 11:00am (1700 GMT) and opened fire.

Images on local television showed the clinic with several blown-out windows, though it was not clear if this was from an explosion or from gunfire.

Buffalo Police Chief Pat Budke told a news conference that Ulrich had a grudge against the medical facility.

"It's a history that spans several years, and there certainly is a history of him being unhappy with ... the health care that he received," Budke said.

But there was "nothing to indicate that we would have been in a situation that we are at today," he added.

Five people were taken to hospital, an official said without disclosing the severity of their wounds.

Budke said there may have been an explosion before the shots were fired.

According to the Star Tribune newspaper the suspect said bombs were on the scene before he was arrested.

Police said they discovered a "suspicious package" while searching the building, and other devices were found in a nearby hotel room where the suspect had been staying.

An eyewitness told Fox 9 that she had gone to the clinic to drop off her mother for an appointment, when two nurses rushed from the facility.

"They said they heard about 11 shots within a minute. They were very scared and just wanted to get out of the place," the woman said.

"About two minutes later, we saw the front windows shot out . . . and we got out of there."

Police said the suspect is thought to have acted alone. AFP