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Ensio Kalevi Koivunen (June 23, 1930, Impilahti - May 27, 2003, Varkaus) was a Finnish serial killer. In the summer of 1971, he murdered three female hitchhikers with carbon monoxide which earned him the nickname "Häkä-Enska" ("Carbon monoxide Enska"). By occupation he was an excavator driver.
In July 11, 1971, 23-year-old Salme Helena Metsänikula (born January 28, 1948) had left from Anjala on bus to her studying location Turku. She managed to get to Koria where she vanished without a trace. She had hopped off at Koria where she had taken a hitch to Turku.
In August 17, 17-year-old Ritva Anneli Raijas (born November 27, 1953) and Pirjo Marjatta Laiho (born July 20, 1954) were hitchhiking from Helsinki's Pitäjämäki to Hyvinkää in order to visit an aunt of theirs. They never arrived to their destination and after a few days, a missing report was filed of them. Last witness of the two was from a bridge in Hämeenlinnantie near Keimola on August 14. A car driver, a 28-year-old Helsinkian man, took the girls with him from the bridge. His memory was based on the fact that the finals of that year's European Athletics Championships on 5000 meter running in Helsinki was broadcast that night on radio.
A Helsinkian family discovered Metsänikula's largely decomposed corpse while on a mushroom collecting trip near the village of Degerby on August 18 covered in moss and sticks and wrapped in plastic tarpaulin with her glasses and neatly cleaned shoes nearby. The victim had lost some of her possessions, such as a camera, three rings and a belt.
Two days later, the police received a report of items that had been found in an old well near Seutula's old airport road. The items were identified as belonging to Raijas and Laiho whose corpses were found in a fashion similar to Metsänikula's. Signs of sexual violence were identified in another one of the girls and both of them had contusions and alcohol in their blood. Each of the victims was killed via carbon monoxide poisoning.
On September 26, the police found a car and apprehended its owner, the then-41-year-old Ensio Kalevi Koivunen. A strand of hair belonging to him had been found from Laiho and Raijanen's murder scene. Additionally a camera found in his house was identified as belonging to Metsänikula and his fingerprints were identified on photos found by a road. He was afterwards sentenced to 25 years in penitentiary after being found guilty of three accounts of false imprisonment, aggravated assault, three negligent homicides and one rape. Koivunen died in 2003 at the age of 72.