Europe's most trafficked inland waterway was closed to shipping on Monday following a weekend container ship accident which left authorities trying to locate the remainder of 31 containers spilled into the Rhine River.

Witnesses on Sunday afternoon watched in disbelief as 21- by 40-foot containers weighing several tons slid off the freight ship Excelsior and splashed into the river near Cologne. The accident apparently occurred after the ship listed to one side. More containers fell off when the boat swung around in an attempt to recover the cargo. The containers quickly began floating down river with the current.

"We were eating lunch, when suddenly the containers tumbled from the ship like building blocks," eyewitness Rita Koch told German radio station WDR.

While 26 of the containers were recovered by Monday morning, a 20 kilometer (12 mile) section of the river will likely remain closed until at least Wednesday as authorities attempt to fish out the remaining five containers, which sunk. Some 100 ships were already backed up on Monday waiting for the river to re-open.

Including helicopters, more than 100 vehicles and watercraft from the police and fire departments worked through Sunday night to clean up after the accident. Before the accident, the ship carried 103 of the massive steel boxes, three of which were loaded with dangerous goods. One such container, full of a mineral oil substance, has already been pumped clean.

Why the captain attempted the disastrous turning maneuver is unclear, but local dailyRheinischen Post reports that a combination of high water, strong current and blustery winds probably contributed to the accident. Mechanical failure has not been ruled out.