hound down
To hunt, chase, or pursue someone relentlessly, until they can be captured or apprehended. A noun or pronoun can be used between "hound" and "down." Police were able to hound down the suspect en route to Mexico.
hound someone or an animal down
to pursue and capture someone or an animal. I will hound the killer down if it takes me the rest of my life. I will hound down that killer if it takes years.