"At this point we have no idea where this has come from, how it's got in there," Paget, 29, said.
''We have scientists on the job, with lawyers, and analyzing everything to try and get to the bottom of this."
Paget said grass, hay and fertilizer will all be assessed for signs of the contaminant.
"I know I did not give this horse Reserpine but we've got to figure out where the result has shown this."
Paget said nothing - including foul play - could be ruled out at this point, ''but I would sincerely like to think that's not happened".
Clifton Eventers owner Frances Stead has spoken out strongly in Paget's defence.
"I'm 100 percent sure, not 99.9 percent, that Jock would never knowingly or intentionally give any prohibited substance to any horse," she said.
"We are all shocked. I don't think you could find a more ethical or honest person, who always operates with absolute integrity."
When Paget won Burghley, fellow New Zealander Andrew Nicholson filled second and third places. Nicholson has been a supportive figure to Paget since the younger man arrived in England to embark on an eventing career.
The world No 1 is adamant - "I believe there's no way he's gone and don't it. Jock is a very genuine bloke."
If the B sample pushes Paget closer to a ban, Nicholson is confident Paget would pull through.
''It's a tough time for him. There's an awful lot of pressure on him at the moment, but you can't take away his ability to ride," he said.