Can I travel to Tibet with travel agent with any kinds of passports?

Anybody who holds common passports can go to Tibet for a visit through any of the local authorized  travel agencies, except the tourists who are holding three different kind of passports which are as below:

1. Those who holds the passports relating to embassy or ambassador or working in there
2. Diplomatic
3. Journalists
Those three kinds of persons can take a trip to Tibet via government office, Foreign Affairs Office, Economic Development Committe, etc. but not through the travel agency.