The prosecution claimed that more than one person must have been involved in the murder because, they alleged, Kercher was stabbed more times than was likely by a single assailant, and the wounds were consistent with the use of more than one knife.
None of that, of course, was the real reason. The real reason was that they had initially arrested Knox, Sollecito, and Lumumba on the basis of crackpot psychological theories, and had egg on their face after having to release Lumumba when he turned out to have an unimpeachable alibi; so they needed Knox and Sollecito to still be guilty in order not to look like the complete morons that they, in fact, were. The bottom line was already written, and it was motivated searching from that point on.
None of that, of course, was the real reason. The real reason was that they had initially arrested Knox, Sollecito, and Lumumba on the basis of crackpot psychological theories, and had egg on their face after having to release Lumumba when he turned out to have an unimpeachable alibi; so they needed Knox and Sollecito to still be guilty in order not to look like the complete morons that they, in fact, were. The bottom line was already written, and it was motivated searching from that point on.