WASHINGTON (CNN) - A preliminary report on the mysterious deployment of an evacuation chute aboard a Barack Obama campaign plane last summer appears to close the book on one potentially embarrassing possibility - that Secret Service agents inadvertently tampered with the chute mechanism during preflight security checks.
"An internal (Secret Service) investigation ... revealed that no (Secret Service) personnel or ... support personnel interfered with or altered the aircraft's ... tailcone evacuation slide," a National Transportation Safety Board report says.
Then-candidate Obama was on the MD-81 charter aircraft at the time of the July 7, 2008, incident, accompanied by his staff, Secret Service personnel, reporters and the plane's crew. An airline mechanic was also on board and was seated in the rear of the aircraft not far from the evacuation slide in the plane's tailcone.
But neither the flight crew nor the mechanic heard the chute deploy shortly after the plane took off from Chicago's Midway airport.
The pilot first noticed that something was amiss when the plane's nose-up pitch continued to increase, even without his input, the safety board said. The pilot regained control using the control column and stabilizer pitch trim inputs.
The plane diverted to Lambert-St. Louis International Airport and made an uneventful landing.
The report says lines of "grease-like material" were found on the slide, seemingly confirming early speculation that the inflated slide had rubbed up against the plane's control cables.
While the report does not give the cause of the incident, it does hint at two possible factors that, acting together, could have caused the chute to deploy.
It says investigators conducted tests in an effort to recreate the incident. In one test, forward tie-down straps were left unsecured. When an inflation lanyard was released the slide inflated, pushing aside its cover in much the same manner as the 2008 incident.
Investigators also said flight recorder data shows the plane took off at a steeper-than-usual angle, and there was sufficient inertia "to allow an unsecured slide cover to rotate open" and inflate the slide.
But safety board investigators generally warn against people drawing early conclusions about the causes of accidents, and spokesman Ted Lopatkiewicz said a final report is expected later this summer.
The evacuation chute mechanism on the McDonnell Douglas plane is unlike those found on many commercial planes. The chute is deployed when a mechanism in the plane's aft bulkhead door jettisons the plane's tailcone. The tailcone falls away from the plane and an attached lanyard pulls open a cover over the evacuation chute, the safety board said. This in turn rotates the slide pack and a second lanyard triggers an inflation cylinder that inflates the slide.
The Obama plane's chute had been inspected one month before the mishap, the report said, and "no anomalies were noted."
After the Obama plane mishap, Midwest Airlines, operator of the charter, released a maintenance bulletin adding an additional check to ensure the security of the slide cover tie-down straps.
Updated at 1:55 p.m. EDT