On March 13, 2004, about 0450 eastern daylight time, a Beech BE90, N11FL, registered to Core Investments Inc. and operated by Execstar Aviation, as a Title 14 CFR Part 135 on demand air taxi flight, experienced a collapse of the left main landing gear during the landing roll at Fort Lauderdale International Airport, Fort Lauderdale, Florida. Visual meteorological conditions prevailed and an instrument flight rule flight plan was filed. The commercial-rated pilot and the six passengers were not injured, and the airplane incurred substantial damage. The flight originated from Cancun International Airport, Cancun, Mexico, earlier that day, about 0100. The pilot stated, during the approach to the airport he selected the landing gear handle to the down position. He observed two green lights and the light for the left landing gear was off on the landing gear indication. He reduced engine power and heard the landing gear warning horn and elected to abort the landing. He informed the tower of the situation and recycled the landing gear. The landing gear indication did not change. He advised the tower he would like to perform a fly-by to have a visual check of the gear. The tower advised the pilot to perform a low approach south of the tower to observe the landing gear position. The pilot cycled the landing gear one more time and got three green lights on the landing gear indication. The tower informed the pilot the gear appeared to be down and in place, and asked the pilot to state his intentions. The pilot than requested a landing clearance. At which time, the tower cleared the pilot to land on runway 13 and called Airport Rescue Fire Fighting to standby. The accident airplane landed on runway 13 and rolled approximately 2,000 feet on the runway before the left main landing gear collapsed. The FAA inspector who responded to the accident site stated, the left main gear assembly was found retracted and up into the engine nacelle. The lock down assembly was unlocked and undamaged. The left main landing gear torque link was broken and the left landing gear actuator gearbox assembly was detached from its mount box. The left main landing gear gearbox drive shaft was disconnected from the universal joint and the connecting bolt was missing. The investigating FAA inspector stated, at the time of the accident the accident airplane had accumulated about 148 hours since its last maintenance inspection, a Phase 3 of its Airplane Approved Inspection Program, on January 3, 2004. The Phase 3 inspection includes checking the actuator support brackets for visible damage, wear, cracks, and loose or missing rivets. An examination of the fractured left gear actuator support box structure brackets revealed a crack that circumscribes the lower hi-shear rivet holes of the outboard bracket. The operator of the accident airplane discovered a structural failure of the left main landing gear actuator support box structure during assessment of the damage to the left wheel well area. They report that it is suspected the structure had at least one existing crack which allowed the actuator to "float" within the support structure not allowing complete spring compression from the actuator to engage the left main landing gear drag leg hooks.