The detainee is suspected of aggravated acceptance of bribes.
The investigation is focusing on PWD-Construction Management, part of the city's Public Works Department that is responsible for public construction projects. Officials at the unit are believed to have taken kickbacks over a period of several years.
Some of the money involved has been recovered. Police say it may add up to around 100,000 euros.
PWD-Construction Management (HKR-Rakennuttaja in Finnish), has more than 110 employees. It purchases design and construction services from the building sector with a value of up to around 100 million euros per year. One of Finland's largest commissioner of construction projects, it has some 400 underway at any given time.