Why was a search warrant issued for Albuera town mayor Rolando Espinosa who is already in jail and why was it served before dawn on a Saturday?

These and more are just some of the questions posed by Interior and Local Government Secretary Ismael Sueno, saying a thorough investigation is needed to get to the bottom of the killing of the town mayor.

Espinosa, mayor of Albuera town in central Leyte province, was shot dead by police after he and fellow slain inmate Raul Yap allegedly fired shots at operatives of the PNP's Criminal Investigation and Detection Group (CIDG) Region 8 who were serving a search warrant at the Baybay Provincial Jail.

In a DZMM interview, Sueno said it is important to find out why a search warrant was issued for Espinosa who is already in jail. The mayor turned himself in to the national police chief in August after President Rodrigo Duterte asked him and his son, Kerwin, to surrender over their involvement in the drug trade.

Sueno said he also wants to find out why there was reported resistance to the entry of CIDG agents to the provincial jail.

"Why was the search done before dawn?" he asked.

The DILG chief said he also wants to find out how Espinosa and Yap got a gun.

He said the National Bureau of Investigation will conduct its own investigation on the incident since an investigation by the Philippine National Police internal affairs service might raise questions.

Sueno: Kung IAS ng PNP lang ang mag-iimbestiga ay baka hindi kapani-paniwala.

— DZMM TeleRadyo (@DZMMTeleRadyo) November 6, 2016

Philippine lawmakers have expressed alarm over the killing of Espinosa inside his detention cell, with the chairman of the Senate committee on justice and human rights, Senator Richard Gordon, describing it as a "slap in the face" of the country's criminal justice system.

Gordon said he may file a resolution to look into such drug-related killings, while Senator Panfilo Lacson was quoted by local media as saying that Espinosa's death was "a clear case of EJK (extrajudicial killing)."

"There's got to be a lot of questions that must be answered," Gordon said in an interview with news channel ANC.

Police said they recovered a .45 caliber pistol and a .38 Super pistol from the cells of Yap and Espinosa. A small sachet containing suspected methamphetamine and drug paraphernalia were also found inside Espinosa's cell, police said.

The presidential palace described Espinosa's death as "unfortunate" and said an investigation was ongoing.

Espinosa had publicly denied any part in the drug trade but said his son was peddling "shabu" (methamphetamine), which he gets from a jailed Chinese drug trader.

Espinosa's son, Kerwin, was arrested by Abu Dhabi police last month, according to Philippine National Police chief Director General Ronald dela Rosa.

In October, Samsudin Dimaukom, a powerful mayor in Duterte's troubled home province of Mindanao, was also killed along with nine of his guards in a shootout, according to police.

More than 2,300 people have been killed in police operations or by suspected vigilantes in connection with the anti-narcotics campaign since Duterte took office on June 30.

Duterte said on Friday his war on drugs had cut back the supply to "very low" levels and thanked China for supporting his crackdown, but swore repeatedly at ally the United States for criticizing it. With Reuters