Inspectors searching for the causes of the collapse of a Minnesota bridge this month that killed at least 13 people have identified pigeon droppings as a possible factor.

It is thought the build-up of guano over many years could have speeded up the rusting of the steel beams in the eight-lane bridge, which collapsed into the Mississippi on August 1. Structural engineers had been aware of the problem since as early as 1987, when inspectors noted a coating of guano on the inside of some of the steel girders.

Pigeon droppings contain ammonia and salt which, when combined with rainwater, can lead to tiny electrochemical reactions that rust steel. "Over a long term, it might in fact cause structural weaknesses," Neal Langerman, of the American Chemical Society, told Associated Press.

The authorities can point to efforts to contain the problem on the Minnesota bridge from 1996, when screens were put up to prevent pigeons nesting in some of the openings in the structure. But the build-up of guano is thought to have continued on other parts of the bridge.

The problem of bird guano is just one of the factors that inspectors are considering as they seek to prevent a repeat of the catastrophe in other ageing infrastructure. About 77,000 bridges across the US have been identified as having the same level of structural deficiencies as the Minnesota bridge.