Police arrested a 51-year-old woman on Wednesday in connection with a string of explosions since January at five locations in Sapporo, including at an apartment building for police officers.

Sanae Nasukawa, who lives in Kita Ward, Sapporo, is charged with causing the blast at the entrance of the police officers’ building earlier this month. It was the fifth in a series of explosions using cooking gas canisters, and more than 2,000 nails were found at the scene.

The Hokkaido police raided Nasukawa’s house on Saturday and have questioned her since.

The first explosion occurred on Jan. 27 at a parking lot of the Kita Police Station, causing slight damage to a car, according to the police. Over the following two months, three similar explosions occurred at a discount shop, supermarket and hardware store nearby.

The fifth blast, which went off on April 3, targeted the apartment building that is also located near the police station.

Investigators were looking into who sent to the police station and media at least four letters criticizing the police and giving advance notice of the explosions.