Firefighters said it happened to the old convent around 2:30 Sunday.
The fire luckily did not spread to nearby buildings, which includes Saint Mary's School.
Firefighters said part of the roof of the convent collapsed as they were fighting the flames.
No one was hurt, or in the building at the time, but witnesses said heavy smoke blanketed the neighborhood.
The convent hasn't been used in years.
The cause of the fire is under investigation.
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