Luckily, Justice Department and FBI will likely have no incentives for investigating that given how they actively positioned themselves in the political debate. So we may never find out.
Still, shame on you.
All you need to do is google. Here is one https://www.cbsnews.com/amp/news/usps-postal-service-employe...
Of course, all 50 states have elections offices that are also tasked with looking for internal fraud. Those offices are staffed by Republican appointees in many of the swing states Trump is complaining so bitterly about. Collectively, they came up with nothing.
Since the Democrats don't control many of the organizations that are supposedly covering up massive election fraud, who do you think is responsible?
Whoever this group is, any plausible conspiracy theory will need to include Democrats, old-school Republicans, and Republicans that are endorsed by Trump.
Good thing we have an administration right now that is actually doing something the previous administration didn't do.
What's lacking is evidence of widespread fraud sufficiently coordinated and systemic to sway the results of a national Presidential election, which we aren't seeing because the system is already set up to monitor for it. 2020 wasn't the US's first time to the election roe-day-oh, and there's been 200 years of infrastructure put in place to detect and punish fraud, malfeasance, and attempts to infringe, dilute, or steal people's right to vote. That's why the claims one candidate made are extraordinary (and they failed to pass a smell test, much less bring actionable claims or evidence that would withstand legal scrutiny).
Most claims we see bandied about online are so risibly ignorant of the existing process that anyone with basic knowledge of how elections work does not take them seriously. They're equivalent in credibility and grasp of the system's machinery itself to saying foreign agents can compromise your computer by infiltrating the 1-bit.
To be clear: I'm excited that people are interested in the process (welcome to the club! There are literally t-shirts!). But I'm disheartened how many people come to the conversation thinking they already know how it works when, no, they don't; like many large and old systems, it has non-obvious quirks and Chesterton's Fences, and common sense doesn't always match up with the how or why of the system. Screaming "fraud" every time one sees something one doesn't understand isn't how one learns; it's how one guarantees continuation of ignorance.