In Julius Caesar, Brutus' servant Lucius finds a letter in Brutus' bedroom. Who does Brutus think it is from? Who is it actually from?

After Caesar’s triumph, there is a high likelihood that he will be established as a King. Several notable citizens and leaders in Rome do not want this to happen. Their reasons are varied, with some being jealous of Caesar’s growing popularity and support among the people. Brutus, who is among those threatened by such a situation, is worried about freedoms already established under the republic. Brutus fears that Caesar will become a tyrant who will...

After Caesar’s triumph, there is a high likelihood that he will be established as a King. Several notable citizens and leaders in Rome do not want this to happen. Their reasons are varied, with some being jealous of Caesar’s growing popularity and support among the people. Brutus, who is among those threatened by such a situation, is worried about freedoms already established under the republic. Brutus fears that Caesar will become a tyrant who will restrict individual freedoms, leading to the destruction of the republic. For this, Brutus is willing to join the other conspirators in their plan to assassinate Caesar.

In Act 1 Scene 3, Cassius and Cinna deliberate on how to fully incorporate Brutus into their plans. Cassius resolves to send an anonymous letter to Brutus to spur him on to completely cooperating with the conspirators. Cassius asks Cinna to slip the sealed letter under Brutus’s window, where he can find it.

CASSIUS: Don’t worry. Good Cinna, take this paper,
And see that you lay it on the praetor's chair,
Where Brutus may find it, and throw this
In at his window. Seal this up with wax
On old Brutus' statue. When you’ve done all that,
Go to Pompey's Porch, where you shall find us.

In Act 2, Lucius, Brutus’s servant, stumbles upon the letter and takes it to Brutus. The contents of the letter lead Brutus to believe that the correspondence is a public protest by the people and that they are seeking his intervention. 

Shall Rome stand under one man's awe? What, Rome?
My ancestors drove Tarquin from the streets of Rome
When he was called a king.—
"Speak, strike, revenge!"—Am I begged, then,To speak and strike? O Rome, I promise you,
If revenge follows, you will receive
Everything you ask for at the hand of Brutus!

In summary, the letter is from Cassius, but Brutus think it is from the people of Rome.