Detectives have declined to divulge the exact nature of her injuries but said the killer tried to cover their tracks by setting fire to her home.
Detective Superintendent Kevin Geddes, who is leading the murder hunt, said: "It is certainly my belief that whoever started the fire did so in order to destroy evidence."
The body of the bubbly and outgoing woman was found at her house in Hazel View on the outskirts of West Belfast after the alarm was raised by a member of the public at 5am on Sunday.
The house was so badly damaged the roof collapsed.
The second floor of the two-storey terrace appeared totally destroyed and witnesses said the flames could be seen from miles away.
Mr Geddes added: "Jennifer, we know was a young mum. Jennifer was very, very popular. Jennifer was very active in the community. At this stage we don't know why Jennifer died or why somebody would choose to kill her."
Ms Dornan, whose two sons and daughter are all aged under nine, was a popular member of the local community.
Hundreds of people attended a candle lit vigil at the scene and an online campaign to raise funds for her family has already exceeded £10,000.
Mr Geddes said: "It goes without saying that her family are devastated, firstly by the news of her death and within the past few hours, the added heartbreak that this was a murder.
"Jennifer leaves behind three young children -- two boys and a girl all under nine. Quite simply they do not understand what has happened to their mum."
Detectives appealed to members of the community who saw her socialising with friends at the Devenish Complex on Saturday night or during the early hours of Sunday morning as she walked home, to contact them.
The group left the venue at Finaghy Road North at about 1.30am and went back to a friend's house.
At 3am, Ms Dornan left to walk the short distance home.
Mr Geddes added: "I am appealing for anyone who saw Jennifer in the bar on Saturday night or Sunday morning or who saw her walking in the Hazel View area between 3am and 4am on Sunday morning to come forward. I also want to hear from anyone who saw anyone else walking in the area around this time, particularly around Hazel View."
Detectives are particularly keen to speak to the driver or occupants of a small silver car which was seen speeding away from the area at the time of the murder.
Anyone with information is asked to contact detectives on the non emergency 101 number.
Sinn Fein MLA Jennifer McCann said the community had been left stunned by the murder.
She said: "This news has sent shock waves through the local community who have rallied round her family.
"I am urging anyone in the community with information which may assist the police inquiry into Jennifer's death to come forward right away.
"This is a time for calm heads and the community needs to support the family and help the police get on with their job of finding Jennifer's killer."
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