Administrative Assistant

I'm an undergrad from Mexico looking for options for a PhD. I've been thinking about writing to some of the professors in the US I'm interested in working with. Some of the professors in my list are very important and in the contact section of their research groups there's an entry for an administrative assistant. I'm interested on writing to them to begin some kind of contact and to ask if they are interested in accepting new students. Should I write directly to the professors I'm interested in or should I write to the administrative assistant? Is the administrative assistant a secretary or what does that title mean?

Thanks in advance,
Jai_Guru

Write to the profs, but don't expect a timely response.

For the really busy profs, CC the administrative assistant.

Thanks,