Israel has blamed Palestinian Islamic Jihad for a deadly blast at a hospital in Gaza City Tuesday night.

Hundreds of people were reported killed in the explosion at al-Ahli Arab Baptist Hospital in Gaza City, which the Israel Defense Forces said was caused by a failed rocket launch by the group. Two U.S. officials also told ABC News the Pentagon independently concluded the blast was likely caused by an errant Palestinian Islamic Jihad rocket that fell short of its target.

Palestinian officials meanwhile are claiming it was the result of an Israeli airstrike.

Like Hamas, Palestinian Islamic Jihad is a Gaza militant group involved in the conflict and has claimed to have taken Israelis hostages. Here's what to know about the group.

What is Islamic Jihad?

Palestinian Islamic Jihad was formed in the 1980s in response to Israel's occupation of Gaza. The radical Islamist movement was founded by Palestinian Fathi Shaqaqi, who was assassinated in 1995, and inspired by the Iranian revolution.

Since its founding, the armed group has grown to be the second-largest militant group in the Gaza Strip and receives support from Iran, Syria and the Lebanese militant group Hezbollah, according to the U.S. government's National Counterterrorism Center.

The group is "committed to the destruction of Israel," and its military wing, al-Quds Brigades, "has been responsible for many attacks on Israeli targets since the 1990s," the National Counterterrorism Center said.

Islamic Jihad vs. Hamas

Both Islamic Jihad and Hamas are militant groups operating in Gaza that are united in their resistance against Israel. Both are also funded by and have close ties to Iran, though function independently, according to the Council on Foreign Relations, a New York City-based independent think tank.

Like Hamas, Islamic Jihad has been designated a foreign terrorist organization by the U.S. and the European Union. Islamic Jihad, the smaller of the two, is considered to be more extreme and is singularly devoted to fighting Israel, while Hamas also has sociopolitical functions as the governing body of Gaza.

Islamic Jihad's popularity has declined in Gaza over its military confrontations and past failed rocket launches, which have resulted in Palestinian casualties, according to the Middle East Institute, a Washington, D.C.-based non-partisan think tank. At the same time, its profile in the West Bank has risen, "in part for attempting to link Gaza to the West Bank but also for its willingness to stand up to Israel, despite taking severe hits," the institute said last year.