How to report suspicious activity of a convicted sex offender

Hi guys I’ve been googling but unable to come up with the right way to proceed on this. My sibling is living in the same apt complex as a convicted child sex offender who acts very suspicious. My sibling has little girls and she’s very concerned about this guys behavior. My sibling feels this person is always lingering around where they live and she finds some of his behavior odd. Is there a place to report this so the authorities can check on this offender and maybe warn him and let him know he’s alarming people? Thanks in advance for any help.