The Federal Trade Commission launched its probe a year ago when Shell Oil Products US announced it was closing the plant, which has since been acquired by unit of Flying J Inc.
The investigation sought to determine if Shell was closing the plant to boost fuel prices.
"After a thorough review of the evidence obtained during the investigation, the commission has unanimously concluded that there would have been no basis under the antitrust laws for challenging the closing of the refinery even if it had not been sold," an FTC spokesman said.
"Indeed, we found that there was strong evidentiary corroboration of Shell's stated reasons for closing the refinery. There was no evidence supporting a conclusion that Shell possessed, acquired, or exercised market power in any way."