To test the capabilities of the Death Note, Light uses six criminals in prison as test subjects. As written in the Death Note, Yadanaka leaves a note before dying of a heart attack in his cell at 6:00pm. The note contains part of a coded message for L.
The Task Force and L learn about the deaths the next day, and the information is added into the NPA database which is accessible through Soichiro Yagami's account. Light is able to access the test results through his father's login the same day.