A former detective who investigated a plot to blow up a Manchester shopping centre said lives were put at risk by a failure to prosecute in the UK.
Retired Det Ch Insp Allan Donoghue said the Crown Prosecution Service (CPS) was wrong not to charge Abid Naseer with planning a terrorist attack.
Naseer, 28, was found guilty by a US court on Wednesday of being part of a trans-Atlantic al-Qaeda conspiracy.
The CPS said the admissible evidence was "very limited".
Naseer had been arrested in Manchester in 2009 during a series of raids by counter terrorism police as part of Operation Pathway.
But the CPS decided not to charge the Pakistani national and, after an attempt to deport him failed, he was allowed back on to the streets.
Later he was issued with a control order and was taken back into custody while the US authorities began extradition proceedings.
Mr Donoghue, the deputy commander of Operation Pathway, said: "The whole command team believed that there was sufficient evidence."
He said the decision not to prosecute had potentially endangered the public, adding: "He [Naseer] was a threat. He was a risk. He had the potential to kill people."
Sir Peter Fahy, chief constable of Greater Manchester Police, said he agreed that Naseer should have been prosecuted in the UK.
"We put evidence in front of the Crown Prosecution Service and that was their decision," he said.
"Because we were a bit unsure about what was going to happen, we had to intervene early to disrupt the plot, but that obviously meant we didn't have all the evidence we might have had later in the investigation.
"It was a really difficult situation. I had to be driven at the time by the need to protect the people of Greater Manchester."
Other serving and former police officers who worked on the investigation, but who have asked not to be named, have also claimed the CPS decision was flawed.
A CPS spokesman defended the decision not to charge Naseer, whose trial at the District Federal Court in Brooklyn heard evidence linking him with declassified documents recovered by the US military during the 2011 raid on Osama bin Laden's compound.
The spokesman said: "The evidence in our possession in relation to Abid Naseer which would have been admissible in a criminal court was very limited.
"Crucially, there was no evidence of training, research or the purchasing of explosives. We had no evidence of an agreement between Abid Naseer and others which would have supported a charge of conspiracy in this country.
"Any decision by the CPS does not imply any finding concerning guilt or criminal conduct; the CPS makes decisions only according to the test set out in the Code for Crown Prosecutors and it is applied in all decisions on whether or not to prosecute.
"The evidence used by the US authorities to extradite a suspect does not need to meet the same tests as set out in the Code for Crown Prosecutors," the spokesman said.
Greater Manchester Police and Crime Commissioner Tony Lloyd said the case was "deeply worrying".
"There are real questions that need to be answered about why Abid Naseer had to be tried in a court in New York rather than here in Britain and I will be raising this issue with the Home Secretary," he said.
Naseer was tried in New York accused of leading an al-Qaeda cell in Manchester.
US prosecutors argued he was also part of a wider conspiracy to carry out bombings in Denmark and America in an attempt to repeat the devastation of the 9/11 attacks.
During the trial undercover MI5 agents gave evidence wearing wigs and make-up to conceal their identities.
One of the spies told the jury he followed Naseer onto a coach from Manchester to Liverpool and observed him watching a video of the Word Trade Centre attacks on his mobile phone.
Defending himself, Naseer insisted he had not been radicalized and that coded emails to al-Qaeda were innocent messages discussing his attempts to find a wife.
He now faces life in a US prison.