the outcome was still difficult to predict.
AMR and US Airways said they would launch a "vigorous and strong defense" and called the Justice Department "wrong in its assessment of our merger."
"Blocking this pro-competitive merger will deny customers access to a broader airline network that gives them more choices," the companies said in a joint statement.
'FULL-STOP INJUNCTION' SOUGHT
Assistant Attorney General Bill Baer, the head of the Justice Department's Antitrust Division, on Tuesday outlined broad objections to the deal based on competition in the industry generally.
"We learned during our investigation about what happened to competition from prior acquisitions, and as we look at the market today, it's not functioning as competitively as it ought to be," Baer told reporters on a conference call.
Baer said he wanted nothing short of a "full-stop injunction" - meaning a court order preventing the merger - even if it means his staff spending months in court.
"We don't file lawsuits unless we're prepared vigorously to defend them, and that's what we're doing right now," he said.
The Justice Department gives parties to a merger several opportunities to respond to likely government objections, allowing them to submit documents and sit down with top department lawyers. The department followed that process in this case, Baer said.
"I don't think it was a surprise to the parties," Baer said of the lawsuit.
US District Judge Colleen Kollar-Kotelly was assigned to oversee the case. In 2001, she was assigned to oversee the penalty phase of the landmark US antitrust case against Microsoft Corp.
An appointee of Democratic President Barack Obama, Baer joined the Antitrust Division in January from the Washington law firm Arnold & Porter. Private antitrust lawyers describe him as a self-assured leader in the field who does not shy from challenging cases.
Under Baer and his predecessors, the division has coined a term - "litigation-ready" - to represent a new willingness to go to court, and it has built up its courtroom experience with the hiring of its first director of litigation, Mark Ryan.
Although there is no hearing date set for the airline case, let alone a Justice Department trial team named, the government's courtroom