Latvia's rescue service says it has finished searching the site of a collapsed supermarket in which 54 people died, including three firefighters, and at least 40 were injured.

The four-day operation was interrupted on Saturday when a large portion of the roof caved in, but no one was injured.

Prime Minister Valdis Dombrovskis on Monday called the supermarket collapse a "serious crime" and said it is not a case where there can be no guilty party.

Latvian officials say 40 people are participating in the investigation, which will likely take several weeks. Possible explanations for the disaster include a flawed design, substandard construction materials, and corruption.