A previous attempt to resupply the fort was made with only a single ship. Sending several gunboats and a large transport with riflemen aboard implies strongly that you are going to reinforce. That, and General Scott's order which said they were going to reinforce is enough to convince a reasonable man that this is indeed their intention.
Besides this, the Lincoln administration had already been stringing them along about giving up the fort, and so they just didn't trust him anymore.
It's likely that the CSA and the locals were acting on rumors rather than gathering reliable intelligence or waiting until the situation cleared up.
MONTGOMERY, April 10, 1861.General BEAUREGARD, Charleston:
If you have no doubt of the authorized character of the agent who communicated to you the intention of the Washington Government to supply Fort Sumter by force you will at once demand its evacuation, and if this is refused proceed, in such manner as you may determine, to reduce it. Answer.
L. P. WALKER.
They were getting intelligence from someone.
Toombsy was right when he counseled against attacking the fort -- if he in fact did -- and wrong when he tried to justify it.
But reading the document makes me wonder if he wasn't lying when he said afterwards that he objected to the attack at the time.
A man can believe both things, especially if he wants to believe both things. I have no doubt Toombsy thought it was a mistake, but afterwards he reconciled himself to believe Beauregard had no other choice.
That first ship was fired on and turned back. So of course you'd send more than one boat the next time.
What kind of ships would actually have been sent to the fort -- as opposed to waiting at sea -- and who would have been on them is a matter of conjecture since the fort was attacked before any ships got there.
And the reference to General Scott is confusing. Are you saying that Confederates had read his communique and not the orders of others in the administration? Otherwise, how does that explain the secessionists response?
The "agent" referred to was sent by Lincoln. Here's Beauregard's earlier message to Walker:
Charleston, April 8.
To L. P. Walker, Secretary of War.
An authorized messenger from President Lincoln just informed Governor Pickens and myself that provisions will be sent to Fort Sumpter[sic], peaceably, or otherwise by force.
[Signed.] G. T. BEAUREGARD.
This document which you did not site, gives a very different view of the situation from what you've said.
DiogenesLamp: " I have no doubt Toombsy thought it was a mistake, but afterwards he reconciled himself to believe Beauregard had no other choice."
Toombsy??
"Robert Augustus Toombs (July 2, 1810 December 15, 1885) was an American politician who was a founding father of the Confederacy and its first Secretary of State."
Toombs was 100% accurate in describing the foolishness of Jefferson Davis' order to assault Fort Sumter.
Toombs' later efforts to explain & justify do not change the fact that Davis had a choice and chose unwisely:
Such logic only applies to a nation already at war, self confessed in this case.