The Nov. 3 assassination of Mohammad Namnam looked pretty much exactly like the fiery deaths of a lot of other Islamic militants in the Gaza Strip over the years. He was making his way in broad daylight through the tattered streets of Gaza City when his sedan turned into a fireball. The missile arrived from an Israeli helicopter hovering so far away that onlookers at first thought the explosion was a car bomb.

The death was not routine, however. Israel has refrained for months from assassination by missile, just as Hamas, the fundamentalist militant group that rules the Gaza Strip, has held back from launching homemade rockets into Israel. And the dead man was a senior operative not of Hamas but of another, more extreme militia called the Army of Islam. Namnam, a senior commander of the group some analysts describe as linked to al-Qaeda, was tracked and killed after Israeli security operatives learned that he was preparing a terror attack on U.S. forces stationed in the Sinai Desert not far from coastal Palestinian enclave ruled by Hamas. (What's behind Gaza's siege mentality?)

But the most striking element of the operation was the source of the tip: Egyptian intelligence gleaned news of the plot from Army of Islam operatives captured earlier in the Sinai. Egyptian security forces work to interdict arms and explosives on smuggling routes that run across the vast expanse from Sudan to Gaza. But sharing the intelligence on Namnam with their Israeli counterparts marked a level of Egyptian cooperation not seen by the Jewish state in years. "Egypt is helping much more," a security source in the region tells TIME. (See Gaza's police force, between Hamas and a hard place.)

This being the Middle East, the explanation involves a blend of shared interests and revenge. Sources familiar with the operation credited the change in Egypt's posture to President Hosni Mubarak's anger at another enemy of Israel, Hizballah, the Shi'a militia based in Lebanon. Last year Egyptian state media announced that 49 Hizballah agents were arrested in Sinai for plotting against Egypt. "They bought apartments near the Suez, speedboats, cars," says the security source. "They built a very big infrastructure around not only Gaza smuggling but also targeting Sinai tourism." Mubarak, incensed, issued a public warning to Hizballah, Hamas and their main state sponsors, Syria and Iran. "We will uncover their plot," the president proclaimed. "Beware of Egypt's wrath."

Egypt and Israel have maintained diplomatic relations since signing a peace treaty in 1979. That treaty returned to Egypt the Sinai peninsula that Israel had captured in the 1967 Six-Day War. It also put in place the multinational force charged with monitoring the desert from a string of outposts and two bases. The Army of Islam plot was aimed at the northern base, called El Gorah, about a dozen miles west of Gaza, apparently hoping to kill Americans. U.S. forces account for almost 700 of the approximately 1,600 military personnel assigned to the Multnational Force and Observers (MFO). Normand St. Pierre, head of the MFO office in Cairo, says Israel and Egypt share responsibility for the forces' security. "The relationship between the countries is really up to them, and I think they know things work better when they cooperate," St. Pierre told TIME, adding that he knew of no specific threat to El Gorah. (See how ruling Gaza is an awkward balancing act for Hamas.)

Israeli sources offered no specifics either, though in announcing the strike on Namnam an Israeli Defense Forces spokesman described him as a "ticking bomb." The dead man was 27, lived in the Shati refugee camp, and was an aide to Mumtaz Dughmush, the leader of a Gaza clan and commander of the Army of Islam. On the spectrum of militant Islam, the group is described as closer to al-Qaeda than to Hamas, which has both embraced and punished the rival. In 2006, Hamas and the Army of Islam cooperated on the capture of Gilad Shalit, an Israeli soldier still held in Gaza. But after Hamas took power of the coastal strip in 2007 it launched an attack against the group, and news reports said Namnam was recently called on the carpet by Hamas for firing rockets into Israel. Hamas suspended rocket attacks after Israel's devastating December 2008 military incursion, which killed more than 700 of its fighters, and a similar number of civilians. (Comment on this story.)

Israeli officials claimed that Hamas was again cooperating with the Army of Islam in the alleged plot against U.S. forces in the Sinai, but offered no evidence to support the allegation.