No Formal Letter after Verbal/Email Offer - Almost a Week! What should I do?

Thought I had sealed the deal upon agreeing and accepting the higher salary but they still have yet to follow up with an official offer and HR hasn't bothered to respond to my once a day messages.

Not sure if they are messing around or found something they didn't like to potentially pull the offer behind my back - didn't even get to do my background check yet.

What should I do: Keep interviewing since things are looking flakey or hold the line since they sent me a bunch of typical onboarding stuff (that I did already)?

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