On November 9, 1999, approximately 1100 mountain standard time, a Beech A60, N457DA, received minor damage when the left engine failed during descent near Boulder, Colorado. The commercial pilot, the sole occupant in the airplane, was not injured. The airplane was being operated on a public use flight by the Boulder County Sheriff's Department under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the cross-country flight that originated from Centennial Airport, Englewood, Colorado, approximately 20 minutes before the accident. The pilot had not filed a flight plan. The pilot said that as he descended to 6,500 feet mean sea level, the engine failed. The pilot landed the airplane without further incident. The engine had 23.5 hours on it since major overhaul. An engine teardown revealed that the #6 connecting rod had failed. Postaccident examination of the #6 cylinder parts, by the National Transportation Safety Board's (NTSB) Materials Laboratory, found preexisting fractures on both the rod cap and the connecting rod. The fatigue features displayed on the rod cap are suggestive of lower stress propagation than those stress propagation marks on the connecting rod. The NTSB's Materials Laboratory found that the two halves of the 2 separated rod bolts from the #6 cylinder (see attached report figures 7, 8, 9, and 10) exhibited surface separation features that were consistent with overstress separations. Additionally, the head portions of these two bolts both displayed bending deformation in an outward direction from the center of the connecting rod and rod cap.