Was Stanton a murder target?

[RJNorton]

How do we even know Booth traveled to Baltimore on April 13th?

I do not see it mentioned in Art Loux's book. (Maybe it's there and I am missing it?)

Mike Kauffman writes, "Someone calling himself "Justice" reported seeing Booth talking to O'Laughlen in Baltimore, and said that O'Laughlen was supposed to accompany Booth to Washington that day. The story has the ring of truth; and makes O'Laughlen's trip to Washington appear less coincidental."

So is this letter from someone named "Justice" the only tangible evidence Booth went to Baltimore on April 13th? Is there more than "a ring of truth" to the alleged trip?