From this NY Times oped:

As this story has unfolded these last few weeks, much has been made of my decision to not prosecute alleged voter fraud in New Mexico. Without the benefit of reviewing evidence gleaned from F.B.I. investigative reports, party officials in my state have said that I should have begun a prosecution. What the critics, who don’t have any experience as prosecutors, have asserted is reprehensible — namely that I should have proceeded without having proof beyond a reasonable doubt. The public has a right to believe that prosecution decisions are made on legal, not political, grounds.
What’s more, their narrative has largely ignored that I was one of just two United States attorneys in the country to create a voter-fraud task force in 2004. Mine was bipartisan, and it included state and local law enforcement and election officials.
After reviewing more than 100 complaints of voter fraud, I felt there was one possible case that should be prosecuted federally. I worked with the F.B.I. and the Justice Department’s public integrity section. As much as I wanted to prosecute the case, I could not overcome evidentiary problems. The Justice Department and the F.B.I. did not disagree with my decision in the end not to prosecute.