DENVER -- Denver police have arrested a 24-year-old man in connection with at least two fires involving Hummer sport utility vehicles in recent days.

An officer arrested Grant Barnes during a routine traffic stop about 11:30 p.m., after finding suspicious materials in his vehicle. He was in the same neighborhood, police said.

A couple in Denver's Cherry Creek neighborhood said their Hummer was engulfed in flames earlier this week, and the flames spread to another car parked nearby.

It's similar to an incident last Saturday when neighbors said they awoke to find Hummer in flames.

According to police, the man is being questioned about other recent vehicle fires.

Police said that the letters ELF left on one of the vehicles was put there several days after the arson. The ELF may be in reference to the Earth Liberation Front, an eco-terrorism group.