Saudi Arabia's civil defense authority says dozens of people have been killed after a crane collapsed on the Grand Mosque in the holy city of Mecca on Sept. 11, 2015.(Photo: 2014 photo by Khalid Mohammed, AP)

Millions of Muslims have made the hajj, the annual pilgrimage to Mecca. But in recent years, the pilgrimage has been marred by tragedies – usually the result of a failure of crowd control or the crush of stampedes.

On Friday, just days before the hajj was to begin, tragedy struck again at the Grand Mosque in Saudi Arabia. Ferocious winds caused a huge construction crane to collapse and crash into the mosque, killing at least 87 worshipers. The Saudi Arabia Civil Defense reported that as many as 184 people had been injured.

The hajj is an annual Islamic pilgrimage to Mecca that is a mandatory religious duty for Muslims. It must be carried out at least once in the lifetime of all adult Muslims who are capable of undertaking the journey. The hajj is considered one of the largest annual gatherings of people in the world. The date of the hajj is determined by the Islamic calendar, which is a lunar year. This year's hajj was set to start from Sept. 21-26.

One of the most recent disasters occurred on Jan. 12, 2006. At least 340 people were killed and 290 injured in a stampede that started when a busload of pilgrims unloaded near a ramp to the Jamaraat Bridge. Pilgrims began tripping over luggage, which resulted in a deadly crush of people rushing to carry out a devil-stoning ritual in the city of Mina, just outside Mecca.. About 2.5 million people from around the world participated in the hajj that year.

In the devil-stoning ritual, Muslim pilgrims fling pebbles at three walls in Mina. It is one of a series of ritual acts that must be performed in the hajj. After the 2006 incident, the Jamaraat Bridge and the pillars representing the devil were demolished and reconstructed. A wider, multilevel bridge was built with massive columns. Each level of the bridge now allows easier and safer access to the columns representing the devil.

Other incidents:

• Feb. 1, 2004: A stampede at the devil-stoning ritual left 251 pilgrims dead and 244 injured.

• Feb. 11, 2003: Fourteen pilgrims died during the devil-stoning ritual.

• March 5, 2001: A stampede at the devil-stoning ritual resulted in the deaths of 35 pilgrims.

• April 9, 1998: At least 118 pilgrims were trampled to death and 180 injured on the Jamaraat Bridge.

• May 23, 1994: A stampede at the devil-stoning ritual killed at least 270.

• July 2, 1990 : A stampede inside a pedestrian tunnel leading from Mecca resulted in the deaths of 1,426 pilgrims.

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