Berlin - Bruno Jost, a former federal prosecutor appointed to examine police conduct in the case, said that a document about Amri's criminal activities ahead of the December 19 attack was manipulated more than a week after his death.
Despite the findings, Jost said that "an undifferentiated and generalized condemnation of the police ... is not justified."
Amri had been under surveillance by German authorities on suspicion of Islamist extremism, but there was insufficient evidence to bring him in on terrorism charges.
However, Amri could also have been detained for lesser offences including drug dealing and assault.
A police file created on January 17 was apparently backdated to November 1, mentioning drug dealing on only a small scale.
Also on Monday, federal prosecutor Thomas Beck told the Berlin parliament that Amri acted alone but received instructions from Islamic State militants operating abroad.
Amri - who drove a steel-laden truck into a Berlin Christmas market on December 19, killing 12 - had planned to die during the attack and had left his wallet and cell phones at the scene to claim responsibility for it, Beck said.