It literally was the federal government's property initially and during negotiations for the transition to the new Confederates they held them hostage via blockade for weeks before opening fire.

You need to negotiate for some peaceful means of transitioning those soldiers and weapons out so the people inside know they won't immediately get killed- otherwise they have every incentive to stay inside -and it's during that process the Confederates opened fire.they would have gotten the fort without doing so- they were literally telling them time and date they would walk out so they would not see their movements as hostile