How does the Wall Street Journal get it's information?

[Frederick]

Robert Mueller’s investigation into possible collusion between the Trump campaign and Russia have reportedly netted its first charges.
It wasn’t immediately clear who could be indicted and what the charges could be; however, the Wall Street Journal reported at least one person could be taken into custody as early as Monday.
The indictment was sealed under orders from a federal judge.

This is what the Wall Street Journal reported:
By Aruna Viswanatha and  Del Quentin Wilber
Updated Oct. 28, 2017 11:45 a.m. ET
At least one person was charged Friday in connection with Special Counsel Robert Mueller’s criminal investigation into alleged Russian meddling in the 2016 presidential election, according to people familiar with the matter.

How does the Wall Street Journal know that?

[Mcjl]

Someone is devulging