Three senior police officers in Qingdao, Shandong province, have been placed under investigation over bribery charges, which are believed to be linked to a major gang case in 2010.

Prosecutors have detained Zhao Min, chief of the anti-terrorism division in the city's public security bureau, and two deputy heads of its crime squads.

More than 50 officials under the city's political and legal departments were implicated following a crackdown on an organized crime group headed by notorious gang lord Nie Lei in September 2010.

Nie, 44, was executed in September last year after he was tried on charges including activities involving prostitution, gun smuggling and deliberately causing injury in Qingdao's largest gang-related case.

After Nie was apprehended, two heads of the police bureaus in the city's Shibei and Licang districts and scores of other police officers were also prosecuted for providing protection for Nie's illegal gambling and drug businesses.

Shan Guowei, head of the police bureau in Shinan district, was tried earlier this month for allegedly taking bribes of 3.75 million yuan ($612,000) from 2004 to 2012 and for protecting Nie's illegal activities in the area.