Simply keep adding photos of persons of interest, particularly groups photos such as launching of events, dinners and other social events. These photos are easily found on news sites, but also in event sections of government websites and in annual reports.
When adding a photo, ensure that you add as much details in the notes/description field as possible, including manually adding the date of the photo if known. This will help you recall and quickly be able to cite the source of the photo, identify people or additional leads for an event. If it is part of a website, don’t forget to archive the page and images using tools such as Internet Archive Wayback Machine.
Google Photos will also import automatically dates, comments and photo metadata from uploaded images. Photos from news articles, scanned photos and those sourced from digital documents such as PDFs do not have this important metadata.
Use photo import and management tools such as gThumb, to edit the dates and add important comment text such as names of events, people, positions and organisations and location. This will be invaluable later, as you try to build timelines about who knew who, and what positions they were holding at the time.
It is also recommended to group sets of photos as albums, by events , cases, sources etc. for example. This will help in getting good search results, especially if some photo metadata such as dates or locations were not added or known.
When there is enough photos for facial recognition Google Photos will start offering you some faces to start assigning names. These are private, and labelling is up to you, even if you may not know their names during initial stages of investigations. Go ahead and label them as Person of Interest A, Malaysian Official 1 or the real names if you know them. You can relabel them at any time.
As you add more photos or a person, from search option of a specific recognized face, there will be dialog from time to time, to help you train and improve matching results.
You can now do searches such as this example for Najib Razak. Or combinations of people such as Najib Razak and Irwan Serigar. If you have added metadata such as dates and locations, you can also use those for your searches.
All the best in your investigations!