Any idea about this initial offer letter? thanks.

I got an initial offer letter in March, accepted the offer by email, and got a welcome email from HR manager. I am still in school, and can only start working in June, so when I asked for official offer letter, they told me they could only start official processing in May. I am a little worried about it, cause i haven\'t signed any official documents. It is a top 50 company in US. Are there any risk? Should I do sth or just wait? Thank you very much for any input.