The security firm that left the fake bomb which caused the evacuation at Old Trafford had assured Manchester United that it had been recovered, the club have said.
United's final Premier League game with AFC Bournemouth on Sunday was postponed after the stadium was evacuated by police following the discovery of a suspect package in the north-west quadrant.
After it had been destroyed by a controlled explosion carried out by a bomb disposal squad, police confirmed that the device had been a training prop accidentally left behind by a security firm who had been conducting safety drills earlier in the week.
United executive vice-chairman Ed Woodward has now placed the firm in question - Security Search Management & Solutions Ltd - under further scrutiny, claiming that every such device, which could not be detected by police sniffer dogs, had been accounted for after its training exercise.
He did, however, praise the response of United security officials and the police in their handling of the situation on Sunday.
"The safety of the fans is our number one aim at every event we host at Old Trafford. Overall, I'm proud of how our staff responded," Woodward said in a statement released by the club.
"The facts are:
o On the discovery of a suspect package, the police and the club worked quickly and closely to identify the threat, make people safe and evacuate the ground calmly and efficiently.
o Fans of both clubs behaved impeccably and the evacuation - the first of its type in the UK - was a complete success.
o Following investigation, the device proved to have been left in error following the training of dog handlers by a sub contractor.
o The contractor had signed the device as having been recovered along with the 13 other devices at the end of the exercise.
o That device could not have been detected by sniffer dogs on the routine matchday search of the 100 Club, as it contained no explosives and was used in an exercise training handlers not dogs.
"Once a live situation was identified, the club and police had no option but to treat the matter as a potential terror threat; we could not have assumed it was a training exercise error. Presented with the same situation in the future, we would take the same action.
"We have worked very closely with the police and counter-terrorism specialists for many years now and enjoy their support on a daily basis.
"For tomorrow's [Tuesday's] rearranged match against Bournemouth, we are working closely with Greater Manchester Police to ensure that robust security measures continue to be of the highest priority.
"We are conducting a detailed evaluation with the help of the police and will share our findings across the rest of the game. Valuable lessons will have been learned from yesterday's events and it is important that those are shared with other stadium operators to ensure that the safety of the public remains the first duty of us all."
Christopher Reid, who runs the security firm, earlier said he was prepared to accept responsibility for the costly mistake.
"I'm sure they're having meetings at the moment to see which guillotine they're going to use on me," he said, as quoted by The Sun. "I have to take responsibility in the end, I won't shirk my responsibility.
"There's a lot I want to say to the fans, but I don't want to do that now. I've actually spoken to a couple of people who were there and they said don't worry about it, everybody was fine.
"I said as long as no one was injured, and I'm really sorry they had to miss their game."