FORT MEADE, Md. — As a military prosecutor held up a knotted bedsheet in court, Private First Class Bradley Manning acknowledged Friday that he fashioned a noose and contemplated suicide shortly after his arrest on charges of engineering the biggest leak of classified data in US history.

The pretrial testimony appeared to support the military’s argument that it was trying to protect the former Army intelligence analyst from harming himself by keeping him in strict isolation, taking his clothes, and shackling him when he was outside his cell.