Banking shares under scrutiny as PacWest leads slide.

US federal and state officials are investigating the possibility of market manipulation behind the recent big moves in banking share prices, as short sellers raked in $378.9 million in paper profits on Thursday alone from betting against certain regional banks. The White House has vowed to monitor "short-selling pressures on healthy banks," while the SEC has warned it will prosecute any form of misconduct. The increased short-selling activity has triggered some calls for a temporary ban, but an SEC official said the agency was "not currently contemplating" such a move.