Dom and his crew actually are the hi-jackers, and they go for "the jackpot" but it all goes wrong, and its up to Brian to save them. Dom and Brian retaliate against Tran and they go and "get them back". After they do, Brian lets Dom escape in his Toyota Supra. Then, (after 10 - 15 minutes of credits,) you see dom driving a chevelle ss in mexico, which is where he ran to. He repeats his saying from earlier on in the film,"I only live life a quarter of a mile at a time. And for those 10 seconds i am free..." as he drives off on an empty trail in Mexico into the sun.
Answer: There are two ideas of why the police did not come around. Firstly they might have found out after the car chase that Brian was an undercover cop so it would be more effective to allow him to try and discover information instead of going around and arresting Dom. The other idea is that the police might not have enough to arrest him. Dom was not the one driving when they found him and other than racing, it might be tough to arrest him.
Lummie ★
Cops are usually completely unaware of an undercover cop's identity. The more, realistic reason is that the cop didn't ID Dom and since Brian's car was later destroyed, there's nothing for them to investigate. Afterwards, they might ID Dom, but there's an investigation on him, so they'd leave him alone.