While details were scant, Prime Minister David Cameron called the killing "truly shocking" and said he had asked Home Secretary Theresa May to call an urgent meeting of the government's emergency committee.

A British government official who spoke only on condition of anonymity because he was not authorized to speak about the investigation said the details that had emerged were indicative of a "terrorist-motivated attack."

May said she had been briefed by Britain's domestic security service, MI5, and by police on what she called a "sickening and barbaric" attack.

Britain's Ministry of Defense said it was urgently investigating reports that a serving soldier was involved in the incident.

Police said armed officers responded to reports of the assault Wednesday afternoon just a few blocks from the Royal Artillery Barracks in southeast London.

Commander Simon Letchford said reports indicated that one man was being assaulted by two other men, and that a number of weapons including possibly a firearm were used in the attack.

Knox, 25, spent four years in prison for the murder of Meredith Kercher while they were exchange students in Perugia, a hilltop Italian university town popular with foreigners. Knox, who became a tabloid sensation in Britain and Italy, was acquitted on appeal in 2011. She returned to her Seattle-area home, but Italy's high court last month ordered a retrial.

In the hotly anticipated memoir, published by HarperCollins and in bookstores on Tuesday, Knox maintained her innocence and wrote that she regretted that in prison she had never reached out to Kercher's family to say that the girl's death had been a "heartbreak to so many."

The memoir was released in conjunction with an interview with ABC's Diane Sawyer that is due to air on Tuesday evening.

Knox wrote that she wished she had reached out to Patrick Lumumba, a Congolese bar owner whom Knox falsely accused of the murder. Lumumba, who was briefly jailed and then cleared, said in 2011 that Knox's accusation destroyed his life.

"Naming him was unforgivable, and he didn't deserve it, but I wanted to say that it wasn't about him," Knox wrote. "I was pushed so hard that I'd have named anyone."

Knox penned letters to both, but her lawyers advised against sending them because it would hurt her legal case.

In the 457-page memoir, Knox writes of arriving in Italy at age 20 with "rookie Italian" and a penchant for "immature antics," eager to develop independence and come into her own.

Knox writes that she and Kercher bonded right away: "We did what all girls do: we talked about the guys we liked in Perugia and the ones we'd left behind."

She denies that the relationship soured and describes a friendly final encounter as Knox and her new boyfriend, Raffaele Sollecito, left the shared apartment hours before the murder.

Knox writes that she did not understand that she was a suspect in Kercher's murder, and believed that she could help investigators find the killer.

"I was too confused to know what the truth was," she writes at one point, after implicating Lumumba and saying she witnessed the murder.

Her "bafflement" at the situation eventually gave way to "quiet indignation and defiance," Knox writes.

"I followed their directions like a lost, pathetic child," she writes of the investigation leading up to her arrest. "I didn't question, I didn't object, I just put my head down when they told me to and trusted that this would all make sense soon."

Knox, dubbed "Foxy Knoxy" in many early media reports, was initially portrayed as promiscuous and dishonest but a lobbying campaign by her family helped change perceptions.

Kercher's half-naked body, with more than 40 wounds and a deep gash in the throat, was found in the apartment she shared with Knox in the town in central Italy. A student at Leeds University, she was 21 when she died.

The Italian high court in March also overturned the acquittal of Sollecito, Knox's former boyfriend, and ordered a new trial on a date still to be set.

The acquittal came after independent forensic experts said the police scientific evidence was deeply flawed and the investigation had been bungled.

An Ivorian drifter named Rudy Guede, was found guilty and sentenced to 16 years in a separate trial. He is now the only person serving time for the murder, although prosecutors say he could not have killed Kercher by himself.

(Reporting by Edith Honan; Editing by Paul Thomasch and Lisa Shumaker)

The EU said the 27 heads of state and governments agreed at summit in Brussels Wednesday to an automatic exchange of bank information among the bloc's tax authorities to catch tax cheats.

German Chancellor Angela Merkel says the agreement long resisted by Austria and Luxembourg marks a "breakthrough" in Europe's fight against tax evasion.

The EU says the policy will be finalized by the end of the year in parallel to negotiations on the same matter with countries such as Switzerland and Lichtenstein which are not EU members.

European officials say tax fraud costs the bloc an estimated 1 trillion euros ($1.3 trillion) a year.