Human rights groups have condemned the way abuse allegations against the armed forces are handled after the trial of seven UK soldiers in Iraq collapses.
Judge Blackett said the evidence was too "inherently weak"
There was not enough evidence for a conviction, a military judge ruled - adding that Army investigators made "serious omissions" during the inquiry.
Amnesty International said courts martial should not be used to try crimes under international law.
Liberty called for the system to be reviewed to ensure abuse is outlawed.
The men from the Parachute Regiment all denied killing Iraqi teenager Nadhem Abdullah in al-Ferkah, southern Iraq, in May 2003.
Judge Advocate General Jeff Blackett ordered the courts martial to acquit the soldiers because of a lack of evidence.
He said the main witnesses had colluded to exaggerate the incident.
But Liberty director Shami Chakrabarti said: "As the death occurred under British jurisdiction we would hope for the highest standards to be employed in the investigation and trial.
"Rather than just trying individuals in these cases, the entire system should be under review to ensure that there is not a culture which would allow the alleged abuses to take place."
Amnesty International's UK director Kate Allen said it had "longstanding concerns" about the way that allegations of abuse by members of Britain's armed forces abroad are investigated.
She said: "International law requires a prompt, impartial, thorough, effective and independent investigation into alleged abuses.
"A decision over whether to bring charges should be taken independently of the commanding officer and other military bodies."
Courts martial were inappropriate forms of justice in cases other than purely military offences, she added, saying they would inevitably considered biased.
"The use of ordinary civilian courts will strengthen national and international public confidence that justice is being promoted in our armed forces."
But defence expert Colonel Mike Dewar said cases like this one, where the incident was alleged to have taken place in a remote part of the desert near Basra, were by their very nature difficult to prove.
He said it would have probably been the word of the soldiers against or two others, and that the prosecution's case was "fragmentary and contradictory".
"As in any court of law if the evidence is not up to mark the it is quite right that they (the defendants) should be acquitted."
He also pointed out that the rules of procedure followed by soldiers in such circumstances were totally different from those which normal citizens or even police officers followed.
"These are military operations and it's very difficult to use the same sets of rules that you and I are used to in every day life."
These were soldiers on a battlefield who may or may not have gone beyond the rules, he said.
It was most likely to be a "cloudy, middle of the road case", he concluded.
But Liberal Democrat defence spokesman Michael Moore said: "The collapse of this court martial raises some very serious questions about the way cases like this are investigated and conducted.
"These issues must be addressed in the forthcoming armed forces bill."