(CNN) -- Swedish prosecutors launched a formal criminal investigation into "unlawful intelligence activity" conducted by the U.S. Embassy in Stockholm, according to a statement posted Monday on the Swedish Prosecution Authority's website.
The statement comes two days after Swedish security police announced they were looking into the activities. The investigation began when U.S. Embassy officials told authorities its employees had been conducting surveillance similar to what was recently discovered in Norway.
Head prosecutor Tomas Lindstrand said the criminal investigation concerns actions taken to protect American staff and the United States' post in Stockholm. The criminal investigation will establish whether crimes have been committed, he said.
The scale of surveillance is not known, but according to a security police statement Saturday, operations have been ongoing since 2000. They were conducted without the knowledge of the Foreign Affairs office, the Justice Department, the security or the local police in Sweden, the statement said.
"I think it's regrettable they have conducted these activities without informing Swedish authorities," Sweden's Justice Minister Beatrice Ask told reporters Saturday, as reported by CNN affiliate TV4.
"I think it is very serious if these activities have been carried out in breach of Swedish law," she added.
The U.S. Embassy in Stockholm released a statement Saturday that recognized the existence of a program to detect suspicious activities around U.S. facilities, and pointed to past terrorist attacks as evidence for why such a program is necessary.
"It is not a secret program, nor is it an intelligence program," the embassy said. "The United States fully respects Swedish law, and welcomes Justice Minister Ask's remarks at her press conference. The U.S. stands ready to answer any questions the Government of Sweden might ask us on this program."
Also Saturday, officials from the Swedish foreign ministry met with the U.S. ambassador to Sweden to discuss the operations, Teo Zetterman, a spokesman for the foreign ministry, told CNN.
"I don't want to go into any details of what was discussed, but it was regarding their surveillance activities," he said.
"Sweden has good ties with the United States, and it is still too early to say whether this will affect our relationship," he added. "We will continue to talk with the Americans about this."
Earlier in the week, authorities in Norway launched an investigation into whether the United States engaged in illegal surveillance in the Nordic country.
The investigation there followed a report by Norway's TV2, which claimed the U.S. Embassy in Oslo has been conducting an "illegal systematic surveillance of Norwegian citizens."
The channel reported the embassy hired former police officers and defense staff to take pictures and register people who behave in a suspicious way in order to stop attacks on American targets in Norway.
Without specifically mentioning TV2, the U.S. Embassy in Stockholm said it was disappointed by the story's impact.
"We regret that inflammatory and inaccurate press reporting which began in Norway about this program has caused unease and concern among some of our friends," the embassy said in its statement.
Last week, U.S. State Department spokesman P.J. Crowley had said Norwegian authorities were fully informed of the program.
A similar kind of surveillance has been reported by media organizations in Denmark and Germany.
Danish TV2 has said the same kind of group operating in Oslo has also been working for a number of years at the American Embassy in Copenhagen. Frankfurter Rundschau, a German newspaper, reported that American and German security experts have monitored activities around the American Embassy in Berlin.