[A]t around 18:30 on Monday, 14 January 2012, large numbers of security officers wearing military uniforms and helmets, and some being masked, stormed a house in Beit Lahiya, where about 20 Palestinians were performing Shiite rituals. The security officers used clubs to severely beat the persons in the house, and then transferred them to the main police station in the northern Gaza Strip. The detainees were placed under interrogation and they were questioned about personal information and the reason why they were in that house. Then the detainees were beaten again and a number of them sustained fractures and bruises as a result. Those who sustained injuries were transferred to Balsam Hospital and Kamal Odwan Hospital.There have been reports that Iran had cut their funding of Hamas because of Hamas' reluctance to take the regime's side in Syria.
This story might mean that those reports are true. Hamas wouldn't dare anger its main sponsor by attacking Shi'ites unless it had nothing to lose.
UPDATE: Avi Issacharoff in Ha'aretz adds:
The Hamas-run government is convinced that Iran is expanding its influence in Gaza by means of Islamic Jihad.(h/t T34)
Gazan sources told Haaretz that Islamic Jihad now contains a group of converts to Shia Islam. The group is led by Iyad al-Hosni, also a convert, who was ousted from Islamic Jihad but recently reinstated, probably under Iranian pressure: Islamic Jihad's leadership visited Iran two months ago, and afterward, al-Hosni was appointed a senior officer in its military wing.
Some of the men arrested on Friday issued a statement on Sunday urging Iran to stop funding Hamas due to its persecution of Shi'ites.
Tehran has already reduced its support for Hamas, among other things because Hamas has refused to support embattled Syrian President Bashar Assad.
Becoming Shi'ite is a growing trend in the Gaza Strip: Hundreds of Sunnis, both Islamic Jihad activists and ordinary people, are known to have converted.