CHARLOTTE, N.C. -- Fire tore through several units at the Sun Valley apartment complex in south Charlotte overnight.

Firefighters got the call to the apartments off Arrowood Road just after 12:30 a.m. Wednesday.

Investigators said the fire started in a first-floor apartment and quickly spread to a second unit. Firefighters had to evacuate other units because the smoke was so heavy.

It took more than 2 dozen firefighters to put out the flames in about 20 minutes, but they still aren't sure what caused the fire. Investigators will continue to search for the cause throughout the day Wednesday.

Families inside the apartments were able to escape with their small children. No one was hurt.

The Red Cross is helping four families find a place to stay.