HELSINKI, Finland — A man who dressed up in a Ku Klux Klan-style robe during an anti-immigrant demonstration on Thursday in southern Finland — an image that drew outrage across the country after it was widely circulated online — was arrested on Tuesday, according to the authorities.

The man, 19, was identified after the protest because he had bought the sheet, cardboard and tape from a local store, the newspaper Ilta-Sanomat reported.

“He was wearing a white robe and a cardboard hat, and had a megaphone in his hand,” said the official, Detective Chief Inspector Martti Hirvonen of the Hame Police Department, which covers an area near Lahti, the southern city where the protest occurred. “Now we are investigating what he said — if he said something which can be understood as racist and inciting hatred against certain ethnic groups.”

The suspect was not detained, but he will probably be charged with desecrating the Finnish flag he was carrying, which is punishable by a fine, Chief Inspector Hirvonen said. The man is also being investigated for inciting people to commit a crime, an offense punishable by up to two years in prison.