It's risky being a so-called protected witness, especially when the targets of the criminal investigations are members of powerful Mexican drug cartels and dirty cops.

The government's witness protection program faced new questions Wednesday after the fatal shooting in a Starbucks of a former federal police commander who turned informant after his arrest last year for suspected drug ties.

Edgar Enrique Bayardo reportedly had been providing Mexican authorities with information on traffickers based in the northwestern state of Sinaloa before he died Tuesday in a hail of gunfire here in the capital.

Federal officials confirmed that Bayardo was a "collaborating witness," but declined to provide more details. Mexico City authorities said Bayardo was hit by at least six bullets when a pair of attackers burst into the Starbucks in a well-to-do neighborhood called Del Valle.

Authorities said a Bayardo associate and another customer were wounded during the late-morning attack. The bloody tableau played out amid a tranquil setting of soft chairs and colorful Christmas decorations.

It was the second time in less than two weeks that a high-profile witness has turned up dead while feeding information to authorities about Sinaloa traffickers.

On Nov. 20, Jesus Zambada Reyes, the 22-year-old nephew of the reputed Sinaloa drug lord Ismael "El Mayo" Zambada, was found dead in a government safe house in Mexico City. Authorities said he hanged himself, but questions swirled over whether he was coerced or killed by cartel hit men.

Investigators often rely on testimony from criminal suspects to penetrate the workings of drug gangs. But the Bayardo killing has reignited doubts about the witness protection program, which was already dogged by criticisms about the trustworthiness of sworn accounts from cloaked witnesses.

Skeptics now question whether Mexico's corruption-ridden law enforcement system can safeguard informants. In particular, the killing stoked speculation about possible leaks by the organized-crime unit of the federal attorney general's office, which Bayardo reportedly had been supplying with evidence on links between the Sinaloa cartel and ranking federal police.

An editorial in El Universal newspaper Wednesday said a lack of transparency in Mexico's court system left it vulnerable to abuse by suspects-turned-witnesses. The Bayardo killing "is one more sign that the federal government should review the use of this resource," it concluded.

Federal prosecutors on Wednesday took over the investigation from Mexico City officials.

Bayardo, a lawyer who had also been a prosecutor in the central state of Tlaxcala, was arrested last year on suspicion of taking $25,000 a month in payoffs from Jesus "El Rey" Zambada Garcia, the brother of Ismael Zambada and father of the witness who died last month. Jesus Zambada and his son were arrested together during a raid in October 2008. Bayardo was arrested days after.

Bayardo and the Zambadas, part of the Sinaloa-based alliance that includes kingpin Joaquin "El Chapo" Guzman, allegedly swapped information about a rival gang, the Beltran Leyvas, and Bayardo was suspected of providing tips on planned police movements.

Bayardo also passed information to American drug agents pursuing members of the Beltran Leyva group, according to reports in the Mexican media.

Bayardo was never formally charged after his detention, and he became a key witness for prosecutors seeking evidence against traffickers and their allies inside Mexican federal law enforcement.

Bayardo's testimony has led to the arrests of at least four senior federal police officials, including the then-chief, Victor Gerardo Garay, according to Mexican news reports.

The daily Reforma newspaper said Bayardo moved among a trio of government-owned safe houses, but recently had been preparing to resume normal life, perhaps by teaching about police issues.

Bayardo allegedly amassed millions of dollars in real estate, jewels, artwork and other property, and was long rumored to have ties to drug traffickers as he ascended to a ranking post in the federal police.