Police were called to Carrington on Wednesday after a member of staff opened a parcel containing an 'unidentified substance'.
The previous day the complex was at the centre of a security scare after a similar package was received.
Police said they believed the same person was responsible for both incidents.
A spokeswoman for GMP: 'Further suspicious packages were discovered on Wednesday May 2 at the Manchester United training ground at Carrington.
'This followed a call on Tuesday May 1 following reports that an unidentified substance had been discovered in an envelope delivered to the address.
'At this stage it would appear that all of the packages have been sent by the same person.
'Tests have been carried out on the substance and it has now been established that it was not harmful. Officers are continuing to investigate the origins of these packages. No-one has been injured during these incidents.'