A soldier stands guard at the police screen as investigators collect evidence at the scene of a blast in a upscale neighborhood in Lahore, Pakistan February 23, 2017.

Pakistan police announced Monday they had arrested 22 people after the lynching of a university student accused of blasphemy, but observers said there was little hope authorities would secure convictions.

A large mob attacked journalism student Mashal Khan last Thursday, stripping, beating and shooting him before throwing from the second floor of his hostel at the Abdul Wali Khan university in the conservative northwestern town of Mardan.

The brutality of the attack, recorded on a mobile phone camera, shocked the public and led to widespread condemnation, including from prominent clerics. Prime Minister Nawaz Sharif vowed to prosecute the perpetrators as protests broke out in several cities.

Salahuddin Khan Mehsud, police chief of the northwestern province of Khyber Pakhtunkhwa, told a press conference the number of people arrested in connection with the case had risen to 22, from 12 at the weekend. They were mainly students but also included some university clerical workers.

He said police had so far found no evidence to support the blasphemy allegations against Khan, and condemned the university for investigating the case without police involvement.

A second senior police officer, who requested anonymity, said many members of the police, prosecution service and judiciary sympathised with the attackers and he did not expect any guilty verdicts.

Blasphemy is a hugely sensitive charge in conservative Muslim Pakistan, and can carry the death penalty. Even unproven allegations can prompt mob lynchings or lesser violence. “There are hundreds of sympathisers in my force and if I take too much interest in the case I might be killed too,” the police officer said.

He added that although arrests had been made on the basis of CCTV footage and video clips, a court would require witnesses to come forward and past experience had shown this would not be likely — partly because Pakistan has no witness protection programmes.

Saroop Ijaz, a lawyer employed by Human Rights Watch in Pakistan, noted that no Muslims were convicted for torching 100 Christian homes in a 2013 incident in Lahore sparked by blasphemy claims, nor for the murder of a young Christian couple a year later.

“Nobody is going to stick their neck out because you will be abandoned,” he said. Vigilantes have murdered 65 people over blasphemy allegations since 1990, according to research compiled by the Center for Research and Security Studies think-tank.

Pakistani Prime Minister Nawaz Sharif vowed to prosecute the perpetrators as protests broke out in several cities.