The phrase "authorities were then" is correct and usable in written English.
It can be used to indicate a sequence of events where the authorities took action or were in a particular state at a specific time.
Example: "After the incident, authorities were then tasked with investigating the cause of the explosion."
Alternatives: "the authorities subsequently" or "the authorities at that point".
The authorities were then able to narrow their focus to the elder Mr. Franklin over the past weekend based on the proximity of his residence to the crime scenes, race, age and other factors.
About a year and a half later there was the second run through the system, the detective said, and the department "learned of a man that as we know now turned out to be a direct relative of our suspect". The authorities were then able to narrow their focus to the elder Mr. Franklin over the past weekend based on the proximity of his residence to the crime scenes, race, age and other factors.
Authorities were then able to go to Raise to see how the scheme generated $7.5 million in proceeds (down from $9 million after the company took its fees and commissions), which was funneled into bank accounts, including ones held at Citi and Wells Fargo.
These authorities were then merged into the Public Transport Corporation in 1989, with the suburban services rebranded as "The Met".
Health authorities were then requested to forward a pack to her general practitioner for subsequent forwarding.
Municipal authorities were then authorised to establish flea markets (tiangge) or vending areas in selected streets, roads and open spaces.
Maybe the authorities are then hoping that it will be forgotten about and everybody will move on.
The Iranian authorities are then able to use the gold to buy dollars and euros needed to purchase other needed imports, or to support the faltering value of Iran's own currency, the rial.
Part of that scheme has been to give money to those local authorities with a resident population that is more than 5% Muslim, that those authorities are then expected to use to fight radicalisation, mostly by giving grants to community groups.
The monetary authorities are then confronted with a harsh choice between violating their announced target, and thus undermining the credibility that was the point of the exercise, or setting policy too tight or too loose, thus doing unnecessary damage to the economy.
Many national football authorities are then expected to follow its lead.
