Defense Secretary Jim Mattis refused to acknowledge the gunman behind the deadly Pittsburgh synagogue shooting as a "man," calling him the "poorest excuse for a man you could ever come up with."

The synagogue attack Saturday left 11 people dead and six more injured, including four policemen. The shooting suspect, 46-year-old Robert Bowers, surrendered to police and now faces 29 separate federal charges, including hate crimes. Of those charges, a majority are punishable by death.

"If there is one person responsible, this individual, I won't even call him a man, he was the poorest excuse for a man you could ever come up with, who would use a weapon in a house of worship on unarmed innocent people," Mattis said to reporters traveling with him to Prague, according to Reuters.

"This is a coward and he is not a man by any definition that we use in the Department of Defense," Mattis added.

[Opinion: I'm a Squirrel Hill Jew, and you cannot break me]

Mattis' comment about there being one "one person responsible" comes as a national debate has arisen about who bears some responsibility for the shooting as well as a flurry of mail bomb attacks against prominent Democrats.

Some critics have pointed to President Trump's rhetoric as being partly to blame, but his defenders have dismissed the association.

“Everyone has their own style, and frankly, people on both sides of the aisle use strong language about our political differences,” Vice President Mike Pence told NBC News. “But I just don’t think you can connect it to acts or threats of violence.”