KABUL, Afghanistan — When Mullah Mujahid, a Taliban commander in Kunduz Province, was arrested last month, there was little reason to think it would have much consequence, either for him or for the government of Afghanistan.

On two previous occasions that Mullah Mujahid had been arrested, tribal elders had intervened and gotten him released. But this time it turned out differently.

Under interrogation, Mullah Mujahid began describing how police officers helped Taliban fighters, sometimes selling them ammunition, other times tipping them off to impending police operations, a member of Parliament from Kunduz, Abdul Wadud Paiman, said in a telephone interview.

Then the captured Taliban leader began naming names, Mr. Paiman said.

That prompted the nation’s spy service, the National Directorate of Security, to undertake a major investigation into the police force in Kunduz, a northern province that has fallen prey to criminal gangs, unaccountable militias and a resurgent Taliban.