PITTSBURGH (KDKA/AP) – Dozens of Civil War-era cannonballs unearthed at a Pittsburgh construction site will be removed by a Maryland firm.

According to the Pittsburgh Public Safety Department, Ordnance Holdings Inc., of Reisterstown, has been hired to remove the cannonballs found near the former site of the Allegheny Arsenal. The arsenal supplied the Union Army, and an explosion there in September 1862 killed 78 people, many of them female employees.

Franjo Constructions, of Homestead, a Pittsburgh suburb, unearthed the cannonballs – believe to be 35 to 43 of them – during excavation for a planned apartment complex.

The public will not be allowed to watch their removal, for safety reasons.

While the removal is in process, Public Safety officials say there will be limited public access to a portion of 39th Street. That begins Tuesday, April 25, and the process is expected to take about five days.

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(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)