From the analysis of the @IDF operational systems, an enemy rocket barrage was carried out towards #Israel, which passed in the vicinity of the hospital, when it was hit.

According to intelligence information, from several sources we have, the Palestinian Islamic Jihad is responsible for the failed launch that hit the hospital. source

There is also some Al Jazeera footage that shows a rocket either failing or being intercepted, and then shortly afterwards a giant explosion on the ground below which is allegedly the Gaza hospital in question.