The agent with the passive voice

The agent is the person or thing that performs the action and is the subject of the active sentence. In most passive sentences, the agent is not mentioned. If it is mentioned, however, it is usually preceded by the preposition by:

The pigeons were dispersed by a tourist walking past. (A tourist walking past dispersed the pigeons.)
He was hit by a falling branch while walking in the woods. (A falling branch hit him while he was walking in the woods.)

The instrument is an object that is not the doer of the action but something that the doer uses in performing the action. If it is mentioned in the passive sentence, it is preceded by the preposition with:

The crowds were dispersed with tear gas. (The police dispersed the crowds with tear gas.)
He was hit with a branch while walking in the woods. (Someone hit him with a branch while he was walking in the woods.)

(In the two passive sentences above, the agent is not mentioned. In the first sentence the identity of the agent is obvious, while in the second one it is unknown.)

With is also often used with the verbs fill, crowd, cram and pack:

The bottle was filled with whiskey.
The hall was crowded with people standing and lying on the floor.
Our room is crammed with furniture.

The preposition in is often used with cover:

When I looked out the window in the morning, I saw that the streets were covered in snow.