I already knew they were not locked in. The goal of the attackers would be to smoke them out and to take them.
There were terrorists watching the ambassador's movements in Tripoli and they probably had plenty of opportunities to kill him before Sept 11 if they wanted to kill him. So it is possible they wanted to capture him instead.
The atackers were pretty hellbent to go as far as trying to break in the mission building and the annex rather than just destroy it; this suggests they were looking to capture or kill people more than just attacking the structure. Most of the time terrorists seem content to just set off a big bomb and run, but this group invaded, like our embassy in Iran in 1979.