28 U.S.C. § 2716: Personal notice

After the first publication of the notice of attachment, a person indebted to, or having possession of property of a defendant and having knowledge of the notice, shall answer for the amount of his debt or the value of the property. Any disposal or attempted disposal of the property, to the injury of the United States, is unlawful. When the person indebted to, or having possession of the property of a defendant, is known to the United States attorney or marshal, the officer shall cause a personal notice of the attachment to be served upon him, but the lack of the notice does not invalidate the attachment.