What does it mean that terrorists will bomb even Muhammad’s burial place?

A suicide bombing outside the mosque where the prophet Muhammad is buried in Medina, Saudi Arabia, rattled the global Muslim community Monday, proving that nothing — not even one of Islam’s holiest sites — is safe from terrorism.

Four Saudi guards were killed and five others were wounded after the attacker detonated his explosive vest in the parking lot outside the Prophet’s Mosque, the Interior Ministry said.

Inside the mosque, thousands of worshipers were gathered for sunset prayer on one of the last days of the holy month of Ramadan.

Islamic State has not claimed responsibility for the attack, but the use of a suicide bomber and the proximity to a mosque indicate that the attack was carried out by someone who either had operational links to the militant group or was at least inspired by it, said Fahad Nazer, a non-resident fellow at the Arab Gulf States Institute in Washington, D.C.

“This attack has made it very clear that ISIS does not seem to believe in any moral red lines whatsoever,” Nazer said, using an acronym for Islamic State. “Even Al Qaeda, which is certainly brutal in its own right, has never targeted Muslims in their houses of worship. ISIS has done that repeatedly.”

Saudi Arabia is an enemy of Islamic State, despite the fact that wealthy Saudi donors have been an important source of funding for the group. Saudi Arabia is the ultimate target for the Islamic State’s expansion because it is the birthplace of Islam and home to its two holiest cities, Mecca and Medina. Sunni-led Islamic State has carried out acts of terror against the kingdom’s Shiite minority as well as its royal family, which the group sees as corrupt and beholden to Western allies.

The Prophet’s Mosque is a major Islamic holy site that millions of Muslims visit every year in conjunction with pilgrimages to Mecca. According to Islamic tradition, Mecca and Medina are supposed to be places of refuge. Muslims believe that before Jesus returns to restore justice, the Antichrist will have free reign over the Earth, but won’t be allowed to enter these cities.

“The sanctity of both Mecca and Medina is one that the overwhelming majority of Muslims believe in. There’s an understanding that an attack on these cities is the gravest sin,” said Nazer.

Within 24 hours, similar attacks took place near mosques at two other locations in Saudi Arabia — one close to the U.S. consulate in Jidda, and one in Qatif, a city with a majority Shiite population.

Maha Akeel, director of the information department at the Organization of Islamic Cooperation in Jidda, said it’s too soon to tell if the attacks were related, but Saudi authorities are investigating the matter.

“These could be ‘lone wolf’ cases with different motives, but they could also be coordinated,” she said.

Saudi Arabia is part of the U.S.-led coalition fighting Islamic State in Iraq and Syria and extremists view the Saudi government to be enemies of Islam. The Interior Ministry reported in June that there have been 26 terrorist attacks in Saudi Arabia in the last two years.

“They, whoever they are, are clearly trying to undermine the security, stability and authority of Saudi Arabia because it has been at the forefront of fighting terrorism,” said Akeel.

“It’s like a stab in the heart of every Muslim,” tweeted Jenan Moussa, a reporter for Al Aan TV, along with a photo that showed a thick cloud of black smoke rising above Medina from the site of the attack.

Leaders from around the world, including Egypt, the United Arab Emirates and Bahrain, spoke out against Monday’s attacks in Saudi Arabia and called for Muslim unity.

Iran’s foreign minister, Javad Zarif, tweeted: “Sunnis, Shiites will both remain victims unless we stand united as one.”

Zeid Ra’ad Hussein, the U.N. High Commissioner for Human Rights said: “This is one of the holiest sites in Islam, and for such an attack to take place there, during Ramadan, can be considered a direct attack on Muslims all across the world.”

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