Video (00:37) : No one was injured and all debris fell within the demolition safety zone, so no passersby were at risk of injury. But investigators are trying to determine what happened.

A beam unexpectedly fell out of sequence in the Metrodome demolition Monday, bringing the teardown to a halt while investigators figure out what happened.

No one was injured and all debris fell within the demolition safety zone, so no passersby were at risk of injury, according to a statement from John Wood, a Mortenson Construction senior vice president.

Crews for St. Paul-based subcontractor Frattalone Companies were working to bring down a section of ring beam in the east-southeast section of the Dome about 1 p.m. when an adjacent section came down out of sequence. No equipment was damaged.

But the incident brought the demolition to a stop pending an investigation.

Demolition requires the same precise engineering as construction and carries similar risks. Surprises are not welcome.

Monday’s unexpected collapse is visible from the street. Some nearby office workers reported hearing a boom. Others say they didn’t hear one, but felt a vibration.

As workers were dismantling the east-southeast side of the Metrodome at about 1 p.m. on Monday, part of an adjacent section collapsed unexpectedly. No one was injured.

A Mortenson spokesman said it was too soon to say for certain when demolition would resume, but possibly later this week.

Since demolition began last month, the Metrodome’s tumble into oblivion has been visible. The east side of the 32-year-old stadium has become a gaping hole. The Dome has been deflated and the building has become a shell.

The biggest bang from the old Metrodome came early this month when crews detonated a dozen charges to sever the support cables, bringing down the whole roof.

Unaccustomed to loud noises — such as from touchdown celebrations — coming from the Metrodome, half a dozen callers reached out to the Minneapolis police wondering what had happened.

The entire Dome is expected to be gone by April, making way for construction of the Minnesota Vikings’ new $1 billion stadium, which is scheduled to open in July 2016.

Excavator Frattalone will cut up and reuse most of the materials from the demolition, including remnants from the roof.

A safety fence now encircles the Dome, keeping the excavators in and the public out. The site, however, is a treasure trove of viewing for fans of demolitions, with heavy equipment crawling all over the site and portions of the Dome’s innards now on display.

The Vikings will play the next two seasons at the stadium on the University of Minnesota campus, but the team is gearing up its sale of tickets in the new stadium that will be twice the size of the Metrodome. In coming weeks, the Vikings will have a fly-through experience set up in their sales office to replicate for would-be season-ticket holders how the new stadium will look and feel.

To buy season tickets — including the preseason games — fans will have to pay $500 to $9,500 per seat at the new stadium. Some 75 percent of the 65,400 seats will be covered by the so-called stadium-builder licenses.

The $100 million in net proceeds from the licenses will count as part of team owner Zygi Wilf’s “contribution” to the stadium construction.