D.C. bar attorney conduct enforcers on Tuesday began presenting their ethics case against two former federal prosecutors who are contesting claims that they withheld a piece of key evidence from the defense lawyers who were advocating for the man charged in the 2001 killing of Washington intern Chandra Levy.

The former assistant U.S. attorneys, Amanda Haines and Fernando Campoamor-Sanchez, have denied violating any D.C. bar rule of professional conduct, and they dispute that any favorable information was withheld at all. The hearing marks a rare exploration of prosecution ethics as it regards the government’s duty to turn over information that defense lawyers can use to poke holes in criminal allegations. The ethics claims were filed last year.