KARACHI- Investigators have found no match for casings of bullets that killed a prominent human rights activist, dashing hopes for quick answers to a murder that has raised fears for the safety of dissenting voices.

Gunmen on a motorcycle attacked activist Sabeen Mahmud late last Friday in Pakistan's southern city of Karachi, as she was leaving her cafe, where she held art exhibitions and talks.

She had just hosted a discussion on disappearances in Baluchistan, a resource-rich southwestern province where the Pakistani army is fighting a separatist insurgency, and, rights workers say, overseeing a campaign of killing opponents.

The army denies rights abuses.

Investigators recovered bullet casings from the scene but drew a blank.

"That suggests that a new group or new weapon has been used in the killing," a law enforcement official involved in the case, who declined to be identified because the topic is sensitive, said late on Monday.

Police say their only witness is Mahmud's mother, who was with her and was wounded. Investigators suspect the killers had a back-up team of two men on a motorcycle and police are poring over CCTV footage.

Investigators desperate for clues are monitoring social media in hopes that loose talk could provide a lead, said another senior law enforcement official.

Authorities had earlier blocked the talk, titled "Unsilencing Baluchistan", when it was scheduled at a different venue.

Mahmud had told friends that officials of the military's Inter-Services Intelligence agency visited her in 2013 to ask about her work and finances, the law enforcement official said.

She had recently asked friends if she should go ahead with the Baluchistan talk, he added.

The army condemned Mahmud's killing, saying its intelligence agents would help in the investigation.

Human rights workers have not been reassured.

"There's a lot of fear among the people, about whoever speaks out about Baluchistan, what's going to happen," said Rukhsana Shama of the rights group Bedari.

"It's easy to point fingers at the agencies but no one knows."