The perfective/imperfective (the one here) difference. In the original sentence, either they were helping the police regularly, or they were helping - and then we have no idea whether they managed to help or failed, maybe they weren't useful to the police in the end.
If they actually helped, succeeded in it, this would be "pomogli". And would denote a one-time help.
Indeed, a mighty thin if any distinction in terms of practical interpretation for a native English speaker. Both versions clearly connote that help was provided in the past, and neither version really distinguishes between a longer or shorter duration, or whether or not it was a one time event or part if an ongoing process, etc..