“This is the tip of the iceberg,” one person told me. “The public has zero idea,” wrote another.

I wrote an article in May about a company that bought access to data from the major U.S. cellphone carriers. My reporting showed that the company, Securus Technologies, allowed law enforcement to get this data, and officers were using the information to track people’s locations without a warrant. After that article ran, I started getting tips that the use of location data from cellphones was more widespread than I had initially reported. One person highlighted a thread on Hacker News, an online forum popular with technologists. On the site, people were anonymously discussing their work for companies that used people’s precise location data.

I called sources who knew about mapping and location data. Many had worked in that field for more than a decade. I also partnered with other Times reporters, Natasha Singer and Adam Satariano, who were looking into something similar. These conversations were the start of an investigation into how smartphone apps were tracking people’s locations, and the revelation that the tipsters were right — selling location data was common and lucrative.

On a big investigation like this one, hours and even days of work can go into a single paragraph or even a sentence. This is especially true in technology investigations because the subject matter is so detailed; combing through data and conducting technical tests is time consuming.

Aaron Krolik, a software engineer on our interactive news desk, helped us test apps. For apps to send GPS data to companies, the information has to be transmitted from the phone to a computer server far away. Aaron used a technical tool that could read and record all that data as it was sent from the phone. He also used more complicated computer security research techniques to help reveal many of the transmissions. We sat in glass-doored meeting rooms in the New York Times building and poked around on apps for hours.