Halliday & Matthiessen (2014: 319):
The line between agent and instrument is not always very sharp. … Nevertheless, there is a significant distinction in the grammar between manner and agency, so that a passive by phrase, if it could not remain unchanged in the corresponding active clause, is interpreted as a participant, not as a circumstance of Manner. This reflects the fact that semantically, whereas the instrument is not usually an inherent element in the process, the agent typically is — although less clearly so when the process is expressed in the passive.