MINNEAPOLIS (AP) - Demolition has been halted at the Metrodome after a beam unexpectedly fell out of sequence and caused a tier to collapse.

John Wood, senior vice president with Mortenson Construction, says nobody was injured in the accident, which happened around 1 p.m. Monday. And he says all the debris fell within the safety zone, so no passers-by were at risk.

Reports say investigators are trying to determine what happened because demolition requires the same precise engineering as construction. The newspaper reports the collapsed tier is visible from the street.

A Mortenson spokesman says it's too soon to say when demolition will resume, but possibly later this week.

The Metrodome is slated to be gone by April to make way for the Minnesota Vikings' new $1 billion stadium.