MANILA - Justice Secretary Vitaliano Aguirre II revealed Thursday how narcotics are smuggled into and cooked inside the New Bilibid Prisons (NBP) in Muntinlupa City.

Citing intelligence reports, Aguirre said methamphetamine or shabu is often mixed with bottled water delivered to the NBP.

Prisoners boil the narcotics-laced water until only shabu crystals remain, he said.

"Iyung pagpasok nila ng napakaraming bottled water diyan, iyun po ay kanilang iinitin at iyung tinatawag nating precipitate, iyung matitira -- iyun po pala ay shabu for internal consumption diyan sa loob," he said.

Aguirre said he also received information that drugs are being cooked inside hidden underground tunnels at the NBP.

The Department of Justice has received offers of technological aid to search for the tunnels.

Since 2015, the government has tried to suppress irregularities like the narcotics trade and the luxurious lifestyle of high-profile inmates inside the country's largest prison facility.

Numerous raids under Oplan Galugad, however, failed to stem the flow of contraband items at the NBP.

President Rodrigo Duterte himself blamed the alleged corruption of the Bureau of Correction (BuCor) for the persistent problem, saying some prison guards accept bribes from detainees in exchange for allowing illegal activities.

Duterte also said drug lords were still able to continue with their illegal activities even while inside the state penitentiary.

Elite policemen from the Special Action Forces (SAF) on Wednesday replaced jail guards in a bid to stamp out the alleged corruption at the NBP.

Earlier today, a batallion of SAF took over security at Maximum Security Compound. @ABSCBNNews @ANCALERTS pic.twitter.com/wH9eVRXEt9

— Raffy Sison Santos (@raffsantos) July 20, 2016

Aguirre said Scene of the Crime Operatives also searched the NBP's Building 14 for contraband items yesterday.

The raid yielded a total of P400,000 in cash, several signal boosters, pieces of Chinese currency and cigarettes with Chinese markings, he said.

The DOJ is now verifying the root source of these items, following Aguirre's directive against simply mounting more raids.

Jail guards, whom Aguirre said are most prone to accepting bribes because of low wages, have also started their months-long re-training.

After completing their refresher course, the guards will be re-assigned to other prison facilities aside from NBP, Aguirre said.