Just to nip the inevitable deluge of “I don’t trust an Israeli news source” comments in the bud, here’s an article from the Navy Times that cited statements by American and UN sources stating the attack took place. No casualties were reported by said US and UN sources.

https://www.militarytimes.com/news/your-military/2024/04/25/us-led-gaza-humanitarian-aid-pier-comes-under-fire-un-officials-say/

At the time of this comment the article states that no group or organization has taken responsibility for the mortar attack.