Montgomery County Fire and Rescue Service officials are investigating what caused a series of brush fires yesterday along I-270 that caused a significant traffic back-up on to the Beltway.

The cause has been ruled “undetermined” by fire and explosive investigators, MCFRS spokeswoman Beth Anne Nesselt said this morning.

Yesterday, a Maryland State Police official told The Gazette the fires might have been intentionally set.

There were about nine separate fires spread almost half a mile from the Rockville Pike ramp to Rockledge Drive. Firefighters were working simultaneously to put out fires on each side of the Grosvenor Lane overpass when the event was called in around 1 p.m.

Firefighters on the scene reported no abandoned vehicles along the right-hand shoulder that may have contributed to the fire.

To access the fire, firefighters and emergency personell had to use all three lanes of the highway, closing off all but one lane of traffic that was getting by in the left shoulder.

That caused back-ups on the outer loop of the Beltway into the P.M. rush hour.