My brother was arrested BEFORE I testified in front of a grand jury, so I'm not sure that the GJ is needed in order to arrest someone.
In my personal experience, my brother had been sexually abusing me for years and I told a teacher about it when I was 14 years old. My brother, who is 5 years older than me, was arrested at the college he attended. I was interviewed multiple times by police with child protective services there, as well. I had a deposition taken. Then I got subpoenaed for the GJ. I testified and my brother was convicted of a class A felony & sentenced.
This was in Washington County (Beaverton... GJ was in Hillsboro) ~ adjoining county to Multnomah.
So, in my experience at least, the arrest came before the grand jury testimony and it never led to a trial but it did lead to a conviction. So... color me :waitasec: as to why some grand jury testimony leads to an arrest/trial and some leads directly to a conviction. I don't understand it at all!