During the night multi-engine instructional flight in visual meteorological conditions, the flight instructor and private pilot receiving instruction reported their destination in sight to an air traffic controller and subsequently started a descent. A review of ground tracking radar information revealed that, following the initial descent, the airplane then climbed at over a 6,000 ft per minute (fpm) climb rate. Two seconds later, the airplane descended from 5,600 ft to 3,000 ft over 11 seconds, resulting in a descent rate in excess of 10,000 fpm. During the rapid descent, the airplane exceeded its published maximum structural cruising airspeed. The distribution of the wreckage was consistent with an in-flight breakup.Postaccident examination of the airframe revealed that the main wing spars fracture surfaces all exhibited a dull, grainy appearance consistent with overstress separation. There was no evidence of any pre-existing corrosion or cracking. Signatures observed throughout the wings were consistent with a positive load separation. Additionally, the stabilator spar fracture surfaces all displayed a dull, grainy appearance consistent with overstress separation. Signatures were consistent with the right stabilator separating in an upward direction and to the left. The left engine fuel selector valve handle was found in the "on" position, and the right engine fuel selector valve handle was in the "off" position; however, the fuel selector valve positions at the time of the accident could not be verified due to impact damage and mount separation at the fuselage, and whether the right engine fuel selector may have been intentionally turned off to simulate an engine failure could not be determined.Examination of the left engine carburetor revealed that two of the three float bays were partially filled with liquid. A float partially filled with liquid would allow the fuel flow into the carburetor float bowl to increase, and would result in a slightly rich condition, particularly at lower power settings. If a rich fuel-to-air mixture had existed, additional signatures would have been present within the engine exhaust and spark plugs, all of which exhibited normal operating signatures. No additional anomalies were found that would have precluded normal operation of the left or right engines.Toxicology testing of the flight instructor reported on the autopsy detected ethanol in liver; however, Page 2 of 14 WPR17FA151testing by the Federal Aviation Administration Forensic Sciences Laboratory did not identify ethanol in the urine. As no ethanol was detected in urine, the ethanol detected in the liver was most likely not ingested; therefore, effects from ethanol did not contribute to the accident. Additional toxicology tests detected evidence of use of the impairing psychoactive drugs marijuana and cocaine; however, no blood was available for testing. Cocaine is a strong stimulant and has significant adverse effects as the drug is metabolized, including agitation, anxiety, inability to focus on divided attention tasks, time distortion and poor balance and coordination. Cocaine's inactive metabolites, but not the active drug, was detected in tissues; it could not be determined when the flight instructor last used cocaine or if he was experiencing impairment from withdrawal symptoms. Additionally, the psychoactive component of marijuana was detected in tissues, but because no blood was available for testing, determination of the instructor's possible impairment from marijuana at the time of the accident was not possible.The circumstances of the accident are consistent with an in-flight loss of control. Whether and to what extent the flight instructor's use of the two psychoactive drugs some time before the accident degraded his performance and contributed to his inability to safely manage the flight could not be determined.