The phrase "are warned by a" is correct and usable in written English.
It can be used when indicating that someone is being cautioned or alerted by a specific source or authority.
Example: "Students are warned by a notice on the bulletin board to avoid the construction area."
Alternatives: "are cautioned by a" or "are alerted by a".
Once there, they are warned by a clairvoyant that John faces great danger if he does not leave Italy.
The interns hear from the firm's LGBT program coordinator, are told of the existence of a "room for transsexuals" at the bank, and are warned by a human resources official, "Don't blog.
They also used the dansa, or balada, a dance song with a refrain; the pastorela, telling the tale of the love request by a knight to a shepherdess; the jeu parti, or débat, a debate on love between two poets; the alba, or morning song, in which lovers are warned by a night watchman that day approaches and that the jealous husband may at any time surprise them.
Demoted and then fired, she was warned by a colleague that her life was in danger.
According to the authorities, Mr. Lattarulo was warned by a consultant that the new foundation was unstable.
Immediately afterwards, Bryant was warned by a reporter from the Sun that "they will get you for that".
The prosecutor, Giuliano Mignini, has said that Sollecito was warned by a tutor at college about watching violent pornography.
When I worked in Belfast, I was warned by a home civil service colleague not to "trust" the Irish government.
Avery appeared to be warned by a teammate, center Chris Drury, to lower his stick during the second-period power play.
She is warned by a male bystander not to buy the woman's wares: the woman is a Tutsi whore, the man says, whose food is likely poisoned.
Can he explain this? "I was warned by a fierce critic, a friend of mine, that it was coming after Soul Provider [in 1989].
