The attack in Tangshan city saw a group of men beating four female diners.
The incident, captured in a shocking viral video, sparked an intense debate about gender violence in China.
A report released on Tuesday found the handling of the case by the deputy director of district police and other officers had been “slow and improper”.
The provincial public security department report did not make clear whether the deputy director – surnamed Li – had been sacked or suspended.
The violence, which occurred in the early hours of 10 June, saw a man approach a woman and put his hand on her back at a barbeque restaurant in Lubei district in Tangshan, a city in northern Hebei province. The woman responded by pushing him away.
The man is then seen striking her in retaliation before other men drag her outside and continue bludgeoning her as she lies on the floor.
The group of men are also seen attacking her dining partners. Another woman who tried to intervene was pushed away before landing on the back of her head against the stairs.
Two of the women were admitted to hospital and were in a stable condition while two others sustained minor injuries, officials said immediately after the attack.
But it has since emerged that the injuries sustained were more serious.
Two of the women spent time in intensive care. The other two women were seen in photos covered in blood, although their injuries were initially classed by police as only “minor”.
Police reached the site at 03:09 (19:09 GMT), almost half an hour after they received the first police call at 02:41, according to a report by state media outlet the Global Times.
The report adds that the suspects fled the scene just minutes after the crime, prompting many on social media to criticise the police response.
Police later said they had arrested nine people on suspicion of violent assault and “provoking trouble”.
The report by Hebei’s provincial public security department said the police had carried out “in-depth investigations into untimely police dispatch… and serious violations of discipline”.
“[The incident] caused a bad impact and aroused widespread concern,” it said.
It added that aside from the police deputy director of Tangshan’s Lubei district, four other police officers were under investigation.
The report also revealed that one of the main suspects was suspected to be involved in a number of other crimes, including illegal gambling, illegal detainment of others and affray.
Police added they were escalating their investigations to look into whether the suspects had ties to criminal gangs.
The viciousness of the attack sparked outrage and heated debate on social media, taking up one of the most-discussed topics on Weibo, a popular social media platform in China, where violence against women remains common.
Many of those posting on social media took issue with the initial official framing of the attack as a simple act of violence, and said the attackers should be punished and given criminal sentences.
Others claimed to have been unfairly treated by gangs in Tangshan and wondered if local police were involved with local criminal figures.
