PHOENIX -- A memo written by a Phoenix police sergeant to his commander accuses two police lieutenants of "serious neglect of duty" and requests that internal investigators review their actions.

Sgt. Phillip Roberts, of the department's violent crimes bureau, details an investigation in which one of his detectives had a lead on someone claiming to have 60 live hand grenades to sell for $200 each.

Roberts claims that with the blessing of his direct superior, Lt. Lisa Messina, his team traced the seller's cell phone to within 100 feet and was prepared to make an arrest.

He goes on to allege in the memo that at that point Messina told him to stop working the case and that she was handing it over to fellow Lt. Lauri Burgett.

Burgett's team never made an arrest in the case, and police later traced the seller's phone to Mexico.

"The suspect is now presumed to be in Mexico and potentially there are now 60 hand grenades on the streets of Phoenix," Roberts wrote. "The neglect of duty I feel has jeopardized the safety of the citizens of Phoenix."

He also wrote, "It is my belief that the plan was stopped because two lieutenants found an opportunity to discredit me."

Messina told CBS 5 News that she wanted to do an interview and dispute the claims made by Roberts; however, she did not show up for the scheduled interview.

Phoenix police spokesperson Cmdr. Chuck Miiller did comment on Roberts' claims.

"The supervisor who had the ultimate responsibility for that investigation said, 'We need to slow down and get some additional facts or maybe some additional avenues for us to pursue,'" said Miiller.

Miiller said that a subsequent investigation, in conjunction with federal agents, concluded that the grenades were likely inert.