A watchdog group said Monday it filed a petition with Denver District Court asking it to compel the district attorney to explain why he decided not to prosecute a Colorado senator for alleged ethics violations.

Colorado Ethics Watch has wanted District Attorney Mitch Morrissey to prosecute Senate Minority Leader Andy McElhany since last year, alleging that the Colorado Springs lawmaker illegally used $2,700 of private donations from a Republican political action committee to create a Web site for the minority office.

"Ethics Watch filed this petition because the public deserves to know why District Attorney Morrissey took over a year to decide to do absolutely nothing about potentially criminal violations committed by a public official," said Chantell Taylor, Colorado Ethics Watch director.

District attorney spokeswoman Lynn Kimbrough said a deputy district attorney did an initial review of the information provided by the group and determined that "provable criminal conduct" did not exist.

The district attorney's office did agree to allow an investigator to do more work on the case. But the investigator also concluded that the case "never rose to the level that we could open a full criminal investigation," Kimbrough said.

"So our ethical obligation at that point is not to go forward with any criminal filing," she said.

The spokeswoman acknowledged the district attorney's office had not sent a letter to the organization explaining the investigator's findings.