Generally speaking, iIf the Marshals had a warrant to search the premises for him or to search it generally, the landlord could have be required to let them in.
Also generally speaking, without a warrant, the landlord should not have let them in, and the marshals should not have gone in.
However, there may be a number of specific circumstances about what the marshals did or did not know, or thought they knew, that would affect the legality of a search like this. If the police think that you were hiding him, you could be under suspicion. You should consider talking to an attorney sooner rather than later.