On October 4, 1999, at 2345 hours Pacific daylight time, a Cessna 421B, N69GP, experienced a collapsed left landing gear during the landing rollout, and subsequently collided with two parked aircraft at the Montague-Yreka Rohrer Field, Montague, California. The aircraft, owned and operated by the pilot, sustained substantial damage. The airline transport pilot and four passengers were not injured. The personal cross-country flight, conducted under the provisions of 14 CFR Part 91, originated at the Reno, Nevada, airport, at 2230, and was terminating in Montague. Night visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he circled the field before landing and noted that the windsock was limp. He stated that there were no other aircraft in the traffic pattern at the time. He reported that everything appeared normal as he setup for the approach to runway 14. All three green landing gear down indicator lights were illuminated. He stated that he made a normal landing, touching down by the runway numbers. During the landing rollout, about 500 to 800 feet beyond his touchdown point, the pilot felt the left side of the aircraft start to drop. He attempted to maintain directional control with aileron and rudder, but the airplane continued to veer left off the runway toward the ramp area. Before coming to a stop, the right side of the airplane impacted the left wing of a parked Cessna P210N, N4768K, causing substantial damage to both aircraft. The Cessna P210N then spun around, came out of its tie down chains, and collided with a parked Cessna 150K, N6316G, inflicting minor damage to it. A Federal Aviation Administration (FAA) inspector from the Sacramento Flight Standards District Office spoke with a mechanic who examined the aircraft after the accident. The mechanic reported that the left landing gear had retracted up into the gear well and there was damage on the outside of the gear door. The nose gear and right gear appeared to be undamaged. The pilot reported that he hadn't experienced any mechanical problems with the aircraft prior to the accident. An FAA certified mechanic looked at the landing gear at the direction of the Safety Board, but was not able to determine whether there was any preexisting damage. The recorded weather data for the Siskiyou County Airport, approximately 5 miles from the accident airport, indicated that the winds at 2355 were from 220 degrees at 10 knots. The pilot estimated that the winds at the accident airport were approximately from 200 degrees at less than 5 knots.