The investigation centres on allegations of tax evasion at cleaning company Capital Städ. It comes after reports that many immigrants are employed as cleaners at McDonalds, Burger King and Max hamburger chains for as little as 4,000 kronor a month.
Prosecutor Martin Bresman told SVT's Uppdrag Granskning programme, which uncovered the alleged irregularities, that several people were suspected of criminal activities.
SVT's report uncovered stories of cleaners working night shifts seven days a week without holidays. They were often working six or seven hours a night. One of the companies named in the report was Capital Städ. A further three companies were also implicated.
Sweden does not have a minimum wage, so the wages themselves, which were as low as 37 kronor an hour, were not illegal.
Nonetheless, the Hotel and Restaurant Union says it has started negotiations with two restaurant chains with an aim of securing damages payments. The union alleges that the companies broke agreements by hiring external cleaning companies without first consulting union bosses.
"The focus has previously been on the cleaning companies, and the hamburger chains have got away lightly. But there is no doubt that they too have been in the wrong," said Håkan Carlsson, communications manager at the Hotel and Restaurant Union to Svenska Dagbladet.
A number of the cleaners interviewed by Uppdrag Granskning were foreign students who needed the money to fund their studies in Sweden. Following the revelations in the documentary, Burger King wrote to cleaners offering them in-house jobs. McDonalds and Max told Svenska Dagbladet that they had not yet received complaints from the union.