Chicago (CNN) -- A Southwest Airlines plane slid off a runway Tuesday during heavy rain at Chicago Midway International Airport, the airline said.
Southwest Flight 1919 had just landed en route from Denver when the Boeing 737-700 ran off the runway, the airline said in a statement posted on its Web site. There were no immediate reports of injuries, it said.
The report is the latest in a series of embarrassing incidents for Southwest Airlines, which came under scrutiny earlier this month when a ruptured fuselage ripped a hole in one of its B-737s at 34,000 feet and then again when reports surfaced that one of its planes came within 176 yards of a small aircraft.
The National Transportation Safety Board said in a statement it was investigating Tuesday's incident.
There were 134 passengers and five crew members on board at the time of the incident, the NTSB said.
"The passengers have been safely deplaned from the aircraft using air stairs and have been bused to the terminal," the airline statement said.
The Dallas-based airline grounded 79 of its 737s for inspection this month after a fuselage rupture punched a hole in Southwest Flight 812 while it was en route to Phoenix, causing a rapid depressurization of the plane. The flight crew conducted an emergency descent, landing at Yuma International Airport. No one was seriously hurt, officials said.
That was followed by a report that a Southwest pilot, responding to a request by an air traffic controller, pulled a plane within 176 yards of a small aircraft to check on the occupants after they failed to acknowledge radio traffic. The controller and the pilot have been suspended while federal investigators determine if the two violated the Federal Aviation Administration's minimums for separation of aircraft.
Southwest Airlines did get some good news Tuesday from the Department of Justice, which approved its acquisition of AirTran Airways. Southwest has offered to buy the airline for $1.4 billion.
The Department of Justice's Antitrust Division said it closed its investigation Tuesday into Southwest's proposed purchase of AirTran, saying it determined that the merger "is not likely to lessen competition."
Southwest Airlines, which reported revenues of $12.1 billion in revenue in 2010, serves 72 cities in the United States. Orlando-based AirTran reported revenues of $2.6 billion. It flies to 69 cities in the United States, Mexico and the Caribbean.
CNN's Justin Lear and Chelsea J. Carter contributed to this report.