Investigators are working to determine what caused an early morning house fire in Montgomery Monday. According to Montgomery Fire Rescue officials, firefighters responded to the 1500 block of South Holt Street around 1 a.m. on a call of a house fire. Once on the scene, firefighters were able to quickly extinguish the flames. No one was home at the time of the fire, and no injuries were reported. Fire investigators say the cause of the fire remains under investigation.