“You did not kill them, but God killed them” reads the banner of the al-Qassam Brigades, an allusion to the militant group's belief that they are carrying out God's work.
The al-Qassam Brigades are often referred to as the military wing of the Palestinian militant group Hamas, which rules the Gaza Strip.
Yet the brigades, which have been behind the majority of attacks on Israel in the last few decades, are often said to act under their own commanders without warning the political leadership ahead of time.
The militant group has fired thousands of rockets into Israel over the last ten years, and has launched hundreds in just the last several days after the killing of their commander, Ahmed Jabari.
The Izz ad-Din al-Qassam Martyrs Brigades, as they are formally known, are believed to have been formed in 1992 during the Oslo negotiations as an effort by Hamas to create a coherent military command.
Others say that the group was formed during the first Palestinian intifada, according to Gulf News.
The group's inception remains mysterious, but The New York Times reported that the organization has gone through a number of stages.
In the early days, when it was run by Yahya Ayyash — who headed the organization in the West Bank and was its chief bombmaker until his death in 1996 — al-Qassam focused on suicide bombings in Israel.
The group began using rockets it called "Qassam" in its attacks on Israel in the early 2000s. In 2006, after Hamas won parliamentary elections in Gaza, the al-Qassam brigades began acting as a police force, while battling rival group Fatah for control.
The al-Qassam brigades defeated Fatah and took military control of the territory shortly after.
Along with homemade rockets, the group also uses longer-range Katyusha or Grad rockets from Iran smuggled into Gaza, which can reach about 40km, said a report by the Washington Institute for Near East Policy. The militant group also imports mortar shells from Iran.
The New York Times reported that typical al-Qassam fighters join the forces by the age of 16, where the teens receive religious indoctrination before being given military training. The organization is believed to number about 15,000 fighters, according to the Washington Institute report.
The al-Qassam operate in loosely-linked cells that are divided by region and by brigade, which are separated into specialties like ground forces, antitank, artillery, etc.
There are, however, other competing military groups in Gaza, and it remains unclear how much control Hamas has over those groups, including Islamic Jihad's Al Quds Brigades and lesser-known Salafist groups like the Army of Islam.
Ahmed Jabari was a commander of the militant group until he was killed last week by an Israeli strike, but was reportedly second-in-command to leader Mohammed Deif, who is now the gravely injured.
The al-Qassam Brigades are named after Izz ad-Din al-Qassam, an imam and a resistance fighter against the British and Jewish population of Palestine, who died in 1935.