Firefighters were investigating a suspicious fire that destroyed three structures in the Agua Dulce area of Santa Clarita Monday morning. L.A. County fire crews were on the scene at 12620 W. Sierra Highway before 4 a.m. after witnesses reported hearing explosions around 3:30 a.m. coming from the garage on the property that houses a $1-million classic car collection. The blaze also spread to surrounding brush, but the flames’ forward progress was held at 3 acres by 6 a.m. and firefighters were monitoring embers to prevent hot spots from flaring up, according to L.A. County Fire Supervisor Robert Diaz.