The Department of Homeland Security is investigating whether an errant shot from Alex Pretti’s handgun — after it was taken off of him — caused Border Patrol agents to think they were being fired on, The Post has learned. As a result of the accidental discharge, which happened while the gun was in a Border agent’s hand, other officers may have believed their lives were in danger — resulting in the split-second decision to open fire on Pretti, killing him, multiple sources familiar with the investigation said. The explanation is a shift from initial claims by DHS officials, who at...