After the 166 passengers were rescreened and the plane was searched, it was given the all-clear and rescheduled for takeoff about two hours later than originally planned.
Sea-Tac Airport spokesman Perry Cooper said the passenger rescreenings and search of Southwest Airlines Flight 149 was due to "security concerns." He declined to be more specific and said the cause is still under investigation by the Port of Seattle police.
"We took the actions out of an abundance of caution," he said.
Wendy Eichinger of Southwest Airlines said the jetliner was expected to take off for Las Vegas about two hours late from Gate B12.
"We thank our Las Vegas-bound customers for their patience and understanding as we put their safety first," she said.
"Because this is a security issue, we won't comment on any specifics except to assure (our) customers and employees that our process works to engage the right experts and secure everyone's safety."