The friar being most suspected knows so when he says this to the Prince and onlookers:
I am the greatest, able to do least,
Yet most suspected, as the time and place
Doth make against me of this direful murder;
And here I stand, both to impeach and purge
Myself condemned and myself excused.
Having confessed he knows things look bad because he is one living man among those newly dead, the friar tells the truth about what has transpired over the past couple of days and Romeo's letter confirms this truth.
Until the Friar professes his guilt, Romeo is suspected of murder since his dagger with blood is found near Juliet. However, after the suspects are gathered and each testifies, with the Friar confessing knowledge of all, there are many who are guilty. For, the true villain in Shakespeare's tragedy, Romeo and Juliet, is the impulsiveness of the characters.
Had Romeo and Juliet not rushed into love, had Romeo not interfered with the quarrel of Mercutio, had Juliet not reacted so hysterically to her father's orders, had the Nurse not foolishly encouraged Juliet to just marry Paris instead of informing her parents, had Friar Laurence not assumed that he could always right things and secretly acted in the marriage and the giving of an elixir to Juliet, had Romeo not assumed that Juliet was dead, and had Friar Laurence not rushed out of the Capulet tomb, assuming Juliet would follow, much of the tragic action could have been averted. The impulsiveness of these characters makes them all culpable for "the story of more woe of Juliet and her Romeo" (V,iii,309-310). For this reason, "All are punished"(V,iii,295).
In this scene, Paris, Romeo, and Juliet are all dead. There are two people who were more or less there at the time. They are Romeo's servant, Balthasar, and Friar Lawrence.
Friar Lawrence himself says that he is the one who is most to be suspected. He says that it is because he is the one who was actually there at the time of the people dying.
The thing is, though, that he was not there when Paris and Romeo died. He came into the graveyard after, where he came across Balthasar. I think that Friar Lawrence says that he is the one most suspected because he feels guilty for having, in a large way, caused Romeo and Juliet to kill themselves.