(CBS) –Every CTA train is being inspected as federal investigators revealed Friday what may have caused the runaway Blue Line train: water.

The National Transportation Safety Board just issued its first report and recommendations following Monday’s collision of two trains.

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Investigators are ruling out sabotage and operator error. But the report also reveals that there was water damage to electrical connection boxes, which start and stop the trains.

The CTA is now inspecting not only 600 similar cars but its entire fleet in an effort to avoid a repeat of Monday’s accident.

The report comes after investigators spent five days pouring over the damaged CTA rail cars. Among the findings: “thermally damaged wiring and water in electrical connection boxes.”

The probe is continuing at the CTA’s Skokie repair shop, under tight wraps.

The NTSB report indicates a concern about the problem affecting other rail cars, too.

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“The CTA,” it says, “has begun inspecting its fleet of similar cars to determine if similar conditions exist on other cars.”

“There is no reason to believe this issue is more widespread,” CTA spokesman Brian Steele tells CBS 2 Chief Correspondent Jay Levine.

The NTSB also recommended the CTA re-evaluate the way it stores out of service rail cars. “Unoccupied CTA trains” it said, “are routinely left powered-up while stored.”

Steele disagrees.

“The CTA does not routinely store its out-of-service cars in a powered-up position. To the contrary, they’re powered down because there’s no reason for them to have power,” Steele says.

Though he concedes the runaway train might have. Among the NTSB recommendations: “De-energize unoccupied CTA trains and use alternate brake systems like wheel chocks to prevent unintended movement.”

The NTSB has not determined exactly what caused the unintended movement –- or the probable cause of the accident. Only the safety measures, like disconnecting power sources and physical wheel locks, would have prevented it.

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Contributing: CBS 2 Political Editor Ed Marshall