Shares of Mesa Air Group dropped to their lowest level in more than four years after a federal judge ruled that the parent of go! misused confidential data to set up the interisland carrier.

Mesa's stock closed at $4.44 on the Nasdaq stock market yesterday, which is the lowest since March 24, 2003, according to Bloomberg L.P. data.

Meanwhile, shares of Hawaiian closed at a six month-high of $4.38 on the American Stock Exchange yesterday.

On Wednesday, U.S. Bankruptcy Judge Robert Faris ruled that Mesa based its decision to start on information obtained from Hawaiian while the Honolulu-based carrier was in bankruptcy and being shopped around to potential new owners.

But Faris made no ruling on whether Mesa should pay damages. The damages will be decided in the ongoing trial, which is entering its second week.

Hawaiian Airlines is suing Phoenix-based Mesa for $173 million in damages, alleging that the airline took thousands of pages of confidential information about Hawaiian's financial projections and marketing plans.

Hawaiian also has alleged that Mesa Chief Financial Officer Peter Murnane deleted hundreds of pages of computer records that would have shown that Mesa misappropriated the Hawaiian information.

If Hawaiian wins its case, Mesa may be forced to shut down go!.

Since the June 2006 launch of go!, Mesa's cash holdings have fallen from about $345 million to about $198 million.

"All that's left now is to calculate the damage," said Sid Levinson, attorney for Hawaiian.

Max Blecher, an attorney for Mesa, called Hawaiian's charges "phony" and said that the records obtained by Mesa were confidential but were publicly available.

Mesa Chief Executive Officer Jonathan Ornstein previously said his company started go! because he thought he could undercut fares charged by Hawaiian and Aloha airlines and still make money. Air fares have fallen in the interisland market since go! began service last year.

Blecher said that Hawaiian Chief Executive Officer Mark Dunkerley is suing Mesa because, unlike previous competitors like Discovery Airways and Mahalo Air, the carrier is very threatening because of its low cost, deep pockets and because "it operated profitably."

"At the end of the day, there isn't any argument that anything we got" was confidential, Blecher said.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com.