Complex sentence: To this day, the humanitarian group is not sure who abducted Mr. Erkel or why, but it does know that he was being shadowed by Russian security forces in the days before his disappearance.
Split into simpler sentences: To this day the humanitarian group is not sure who abducted Mr. Erkel or why. But it does know that he was being shadowed by Russian security forces in the days before his disappearance.