USA Softball - Interference - Thrown ball

[EricH]

Getting ready to umpire my eleventh national next week, and I still have questions here and there.

Situation:

One out, runner on first (R1), batter (B1) hits a ground ball to F3, who throws to F6 at second base to turn a double play. The thrown ball hits R1 in the shoulder. In the umpire's judgment, F3 did not intentionally throw at R1, and R1 did not intentionally interfere with the throw. Both R1 and BR arrive at 2nd and 1st base safely before F4 retrieves the ball.

What is the result of the play?

The rulebook states only that a runner shall be declared out for interfering "with a thrown ball" - no mention of intentionality (thus the reason for my question).