Her attacker, 34-year-old Matthew Williams, who had recently been released from prison, also died at the scene shortly after police discharged a 50,000 volt Taser and arrested him.
His family told the BBC they were "devastated by the death of an innocent young lady".
"Our thoughts and prayers at this time are with the victim, her family and friends.Words cannot express our grief."
Gwent Police said that while under arrest Williams became unresponsive and officers and paramedics administered first aid but he was pronounced dead.
As well as the murder investigation, the Independent Police Complaints Commission (IPCC) watchdog has launched its own probe following the man's death.
An IPCC spokesman said: "The man who who died in the incident near Blackwood has now been formally identified as 34-year-old Matthew Williams.
"Mr Williams was a resident of the Sirhowy Arms Hotel in Argoed.
"The IPCC is investigating the circumstances surrounding the death of Mr Williams. A post mortem is being carried out today."
The IPCC added it was usual practice for this kind of an investigation to take place following any death in police custody.
Officers are treating Miss Yemm's death as murder and are not looking for anyone else. Her family, and officers who attended the incident, are being supported by specialist officers.
Police have refused to comment on the specifics of the incident, but sources confirmed the victim suffered substantial facial injuries.
A spokesman said: "The murder investigation is continuing today, officers and crime scene investigators remain at the scene. To confirm, we are not looking for anyone else in connection with the incident.
"We are co-operating fully with the IPCC, which is independently investigating the death of the man. At this time our thoughts are with the families of the deceased following this extremely tragic incident.
"Our officers who attended the incident yesterday are being fully supported by our force occupational health department."
Speaking to reporters today, Chief Inspector Paul Staniforth said: "The post-mortem for Cerys will start today, but will take some time.
"Speculation about what happened at the scene and the cause of death is unhelpful and very upsetting for the family of the deceased."
Yemm, who worked in sales at Next, was described as a fun-loving young woman who had been at a Halloween party with friends just days before the attack.
It is believed she met her attacker through mutual friends.
Friends of Yemm as well as locals expressed their shock at the news.
Gareth Griffiths, who lives in Argoed, said: "We heard on the news that it has been cannibalism. Nobody wants to hear that, it must be a horrific way to die, it's horrible, it's unbearable."
A friend of Yemm, who did not want to be named, said: "I feel stunned, shocked and sick to my stomach. It's horrific. She was a lovely person. She didn't deserve to die like that."
Scores of tributes have been paid to Yemm - whose family and friends have been left devastated by her death.
Her heartbroken family are being looked after at their home in nearby Oakdale by specialist officers - who said her relatives were too upset to talk.
A friend of the former Oakdale Comprehensive School pupil, who asked not to be named, said: "To say everyone is shocked is an understatement.
"I wasn't able to sleep last night thinking about what happened. It is beyond horrific. I feel sick to my stomach.
"She was so lovely. Smart, attractive and from a good family too.I just can't get my head around it."
The Sirhowy Arms Hotel, where the incident unfolded, was a popular local pub until it was turned into a B&B catering for up to 26 guests.
The redbrick hotel has nine rooms - five doubles, three singles and a family room. But over the last few years it has been used by Caerphilly Council to provide temporary accommodation for homeless people.