Dutch police say three farmworkers have died and a fourth has been critically injured after they fell into a silo filled with manure for use as fertilizer in the northern town of Makkinga.

Provincial police in Friesland, where the town is located, tweeted that it wasn't clear what caused the fall Wednesday. The victims' identities won't be released until their families have been informed, police spokesman Paul Heidanus told local broadcaster Omroep Fryslan radio.

In a story written in the Frisian language spoken by natives of the area, Omroep Fryslan said emergency services including firefighters, medical helicopters and a diving team were assisting at the scene.