What an odd question.
"only" here is badly placed as it could (admittedly unlikelily) be attached to "pursue", so that it would appear that you were pursuing one of the parties, to the exclusion of all other activities. Here it is intended to mean that you should go for both sides, and although "one of the parties only" is strained, "you must pursue both [or all] of the guilty parties" would serve better.
yeah that's true...
you should be fair when it comes to that kind of matter.
Pursuing both parties means that you are fair in the case that your are holding. Interviewing the two parties in a fair manner, no one is right and wrong. They have a freedom of speech and declaring thier alibis to defend themselves....