Well, you said they've shared info with the guards. Have the guards follow up and investigate what they've been told. They need to verify what they've been told, and would be worried that a bunch of amateur sleuths might have missed something important.

So have them question witnesses and suspects that the PCs already interviewed. Investigate areas the PCs already cleared out. If the PCs ask questions, explain that the guards are making sure that they didn't miss something as they are not trained invesigators, but also make it clear that the guards are grateful for their help.

Maybe mention casually that guard patrols have increased. That might also encourage the PCs to continue to work with the guards. If there is a guard nearby when they find something important, they may immediately ask them for assistance.