WAUSAU, Wis (WSAU) Fire investigators are trying to determine what caused Sunday's blaze at the Tamarack Heights Apartments on Wausau's far-west side. The fire started in an upstairs apartment on the east-side of the building. It started near a fireplace. Neighbors say the renter in that unit had built a fire earlier that evening.

The fire spread into the walls and ceiling. The roof partially collapsed. Firefighters described a wall of flames by the time they arrived. Several renters woke up other tenants to make sure they got out. No one was injured.

The fire caused at least $300,000 worth of damage. The cause is still being investigated.

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A few hours later, two suspicious grass fires were reported on Wausau's east side. Firefighters believe they were intentionally set along the train tracks near Northwestern Avenue.

A small wooded area was blackened. No structures were damaged.

Police and firefighters are investigating a series of arsons that date back to last June. Two of the fires involved abandoned buildings.