The phrase "arrested them before" is correct and usable in written English.
It can be used in contexts where you are discussing a situation in which someone was apprehended prior to a specific event or action.
Example: "The police arrested them before they could escape the scene of the crime."
Alternatives: "detained them prior to" or "apprehended them ahead of".
The local police arrested them before they could make much of an impact.
"The police, led by supervising officers, stopped peaceful protesters on the sidewalk, surrounded them with a blue wall of police, told them to disperse, and then arrested them before they possibly could," Wylie Stecklow, an attorney for the protesters, said in a statement.
He was promptly arrested, before being released pending a court appearance in September.
The alleged plot was similar to other cases in in which the F.B.I. had secretly worked with people who were said to have indicated a desire to commit an attack, only to arrest them before it could be carried out.
(When they tried to return, a highway patrolman guarding a checkpoint had threatened to arrest them before finally waving them through). "Everything is almost in place, where I want it to be," Kurt said.
One told us: "The street crime initiative has arrested hundreds of people and charged most of them, but we have probably arrested them 15 times before.
They then chained themselves to the camp's front gate before authorities arrested them.
The police waited until the artists finished the mural before arresting them, to avoid angering the crowd that had assembled to mourn, police officials said.
In the next few years, the D.E.A. lured two of the most wanted arms dealers in the world, Monzer al-Kassar and Viktor Bout, into drug-related conspiracies before arresting them, in Spain and Thailand, respectively.
As Donald and Tracey popped champagne to celebrate their son Tim's 20th birthday, Swat teams swarmed over their home in Cambridge, Massachusetts, before arresting them on suspicion of being unlawful agents of a foreign government.
Indian police paraded a man and his son in "space suits" before arresting them for allegedly defrauding a businessman by pretending to work for Nasa.
