I am a PhD student in the US and teaching a lab this semester. The labs have become online and students are provided with data and submit their reports. All communications take place through email. There is also online office hours but relatively few students attend.
Recently I found two groups submitting the exact same drawing in one of their lab reports (this was a significant part of their work for the report). So I emailed them separately and asked whether this report was completely their own work. Or was it created by one of them and shared for others, and if not, where was it taken from. One of them did not reply at all. The other did, offering apologies but saying that we did not plagiarize. But they also didn't reply to my next email in which I told them exactly which part has been found in another group's work. Basically, they didn't say anything in the face of clear evidence. I should mention that I did have email communications with the same students before and I'm sure they have read my emails.
If it was an in-person class I could just talk to them in the next session to clarify things. But now, they're just ignoring my emails. This has left me with my own speculations. The most probable scenario is that both have used material from the previous semester, and since they neither denied, clarified, or apologized for this, I have lost confidence in their other reports too.
This is making it difficult for me to make an appropriate (proportionate) decision and honestly, I don't know what to make out of this. Are they too stressed/ashamed to reply or are they so relaxed about this that don't even bother to do so?
How should I proceed from here? Is it OK for students to ignore the instructor's emails on such an important issue? And should I continue grading their reports? As I said, I can't trust their works anymore even though I don't have evidence of further plagiarism elsewhere. Would it be appropriate or too much to give a failing grade because of a single verified incidence of plagiarism?