The family of murdered student Meredith Kercher have said the search will go on for her killer, after an Italian jury acquitted Amanda Knox and Raffaele Sollectio.

Lyle Kercher, Meredith's brother, said Monday had been a "long and difficult day".

"While we accept the decision that was handed down and respect the court and the Italian justice system we do find we are left looking at this again, and thinking how a decision that was so certain two years ago has been so emphatically overturned," he said.

He said the family now needed to know who was responsible along with Rudy Guede for the death of his sister. The initial verdict had concluded that Guede had not acted alone.

"If the two who were released weren't the guilty party, we are wondering who is the other person or people?" he said.

"It's back to square one, the search goes on"

He also expressed his gratitude to the support his family had received throughout their ordeal and dismissed suggestions that the case pitted the UK against the United States.

"We're really grateful for support we've had, not just from Italians, a lot of people from around the world including a lot of Americans," he said.

"Some people in the press and people on blogs and forums have tried to create some sort British-American-Italian divide and really that's nonsense."

The Kercher family were speaking in Perugia, Italy, as Knox prepared to head home to the United States. She is due to fly on a commercial plane to Seattle via London.

Knox also thanked the support she had received. "I will always be grateful for their courageous commitment ... (grateful) to those who wrote to me, to those who defended me," she said in a letter published on Tuesday.

As the Knox family celebrated in the US, Stephanie, Meredith's sister, said the family could not find peace until they knew the truth about what really happened.

"Until the truth comes out we can't forgive anyone," she said. "We've said all along we don't want the wrong people put away for a crime they didn't commit," she said.

"It was a bit of a shock. We were prepared for all the outcomes possible. You don't know how you're going to feel or react. It's very upsetting either way. We still have no answers."

Meredith's mother, Arline, said no one involved had been left "untouched" by the death of her daughter.

"What happened to my daughter Meredith is every parent's nightmare of something so terrible happening," she said.

Meredith was found dead in 2007. Her throat had been slit and her semi-naked corpse was partially covered by a duvet.

"Basically she was in the safest place, her bedroom, and a student who was studying at the Italian university," she said. "Nobody is untouched by this."

Knox and Sollectio were acquitted of Meredith's murder on Monday. Both defendants made emotional pleas in the Italian court protesting their innocence.

Speaking in Italian Knox told the court that she was not at the scene of the crime and had unfairly spent the last four years in prison.

"I've had to deal with unfair charges and unfounded charges. I am paying with my life for things I did not commit," she said.

"I have never done what they say I did. They said this how things go, this is not so. I had good relationships with everybody who was living in my flat. I was untidy, careless, but we did have good relationships.

"We were all willing to help each other. I shared my life particularly with Meredith. We were friends."

Sollectio also told the court he had nothing to do with the murder.

"I've never harmed anyone never, ever in my life. The charges that have been made against me all these years were so out of place somehow i thought they would disappear," he said.