Can someone please explain how to catch the mastermind?

I just tried everything I could think of but couldn't even get a clue about the mastermind...

Any help/hint is appreciated. Thank you.

It can be tricky.

There are a few things to do.

First if you know what organization they are with.

Go into the Data and look for Organizations
See if any cities you've been tracking are listed for that organization, if so they have an office there in that city, bugging those offices may lead to additional information.
Also look for allies and associates.
Make a note of those, if you know the lcoation of an ally or associate in the same city you believe the mastermind is in, bug those offices. Watch one. Track Vehicles. Bug any new buildings you are lead to then wait and follow again (I had to do this twice in the mission I just finish...it led me to where the mastermind was).
Just takes perserverance really, hit every office you can until you find information you need.
Good Luck

To catch a mastermind, you have to know where he is and break into that place and arrest him. Bring lots of assault gear.
To know where he is you need to know the city he lives in and the organization he belongs to.

You can find out these two things in several ways. Here are a few (or a lot?).

1. In some plots, one of the subordinates may send a message to the mastermind. Intercept and decode that message, and you've probably got the location and/or organization. Not all plots have these messages.
2. Some plots involve getting things to the mastermind. If you let the plan succeed, you might trace the items back to the mastermind. This is an expensive way, but if you've already bungled a case, keep all couriers, messengers, etc under surveillance and you might at least get the target city. If a courier traveled somewhere to deliver some stolen goods, that somewhere is probably the city where the mastermind lives.
3. When you complete a case, sometimes you get some data about the mastermind. You'll get a name first, but as you solve cases you will eventually get the city and/or organization.
4. If you've got a name, ask a computer. Input the name of the mastermind on any computer you can and see if you get a match. This usually works for computers in the same organization. If you know the name and organization, break in and use the computer to find out the city.
5. Sometimes (very rarely) you might get a clue pointing to a criminal mastermind. Follow these clues.
6. When wiretapping or breaking in, take note of the names mentioned in passing in captured data.You should play the game with a notepad at hand for this and other types of important data (like double agents). The names might be used to try computer searches in a case. More importantly, keep the last case's list at hand. If in the next case you have a coincidence, that's the name of the mastermind.
7. When seeing the chronology at the end of a case, grab the manual. The mastermind is in shadows, but you can try to match the head contour to the pictures in the manual (they match the drawings!). That way, you can find out which organization he belongs to.
8. Use the name to try to narrow the org list. As an example, Omar Khaled is going to belong to an arab organization, so don't bother looking for him in the Stasi.
9. If all fails, turn as many participants as possible and set the plot back as far as you can. This can buy you time to hop around from place to place looking for the mastermind while the plan unfolds. This is also a great tactic to hunt all the double agents.

Enjoy!