The attack Saturday in the Syrian desert near the historic city of Palmyra killed two U.S. service members and one American civilian and wounded three others. It also wounded three members of the Syrian security forces who clashed with the gunman, interior ministry spokesperson Nour al-Din al-Baba said.

Al-Baba said that Syria’s new authorities had faced shortages in security personnel and had to recruit rapidly after the unexpected success of a rebel offensive last year that intended to capture the northern city of Aleppo but ended up overthrowing the government of former President Bashar Assad.

“We were shocked that in 11 days we took all of Syria and that put a huge responsibility in front of us from the security and administration sides,” he said.

The attacker was among 5,000 members who recently joined a new division in the internal security forces formed in the desert region known as the Badiya, one of the places where remnants of the Islamic State extremist group have remained active.

Attacker had raised suspicions

Al-Baba said the internal security forces’ leadership had recently become suspicious that there was an infiltrator leaking information to IS and began evaluating all members in the Badiya area.

The probe raised suspicions last week about the man who later carried out the attack, but officials decided to continue monitoring him for a few days to try to determine if he was an active member of IS and to identify the network he was communicating with if so, al-Baba said. He did not name the attacker.

At the same time, as a “precautionary measure,” he said, the man was reassigned to guard equipment at the base at a location where he would be farther from the leadership and from any patrols by U.S.-led coalition forces.

On Saturday, the man stormed a meeting between U.S. and Syrian security officials who were having lunch together and opened fire after clashing with Syrian guards, al-Baba said. The attacker was shot and killed at the scene.

Al-Baba acknowledged that the incident was “a major security breach” but said that in the year since Assad’s fall “there have been many more successes than failures” by security forces.

In the wake of the shooting, he said, the Syrian army and internal security forces “launched wide-ranging sweeps of the Badiya region” and broke up a number of alleged IS cells. The interior ministry said in a statement later that five suspects were arrested in the city of Palmyra.

A delicate partnership

The incident comes at a delicate time as the U.S. military is expanding its cooperation with Syrian security forces.

The U.S. has had forces on the ground in Syria for over a decade, with a stated mission of fighting IS, with about 900 troops present there today.

Before Assad’s ouster, Washington had no diplomatic relations with Damascus and the U.S. military did not work directly with the Syrian army. Its main partner at the time was the Kurdish-led Syrian Democratic Forces in the country’s northeast.

That has changed over the past year. Ties have warmed between the administrations of U.S. President Donald Trump and Syrian interim President Ahmad al-Sharaa, the former leader of an Islamist insurgent group Hayat Tahrir al-Sham that used to be listed by Washington as a terrorist organization.

In November, al-Sharaa became the first Syrian president to visit Washington since the country’s independence in 1946. During his visit, Syria announced its entry into the global coalition against the Islamic State, joining 89 other countries that have committed to combating the group.

U.S. officials have vowed retaliation against IS for the attack but have not publicly commented on the fact that the shooter was a member of the Syrian security forces.

Critics of the new Syrian authorities have pointed to Saturday’s attack as evidence that the security forces are deeply infiltrated by IS and are an unreliable partner.

Mouaz Moustafa, executive director of the Syrian Emergency Task Force, an advocacy group that seeks to build closer relations between Washington and Damascus, said that is unfair.

Despite both having Islamist roots, HTS and IS were enemies and often clashed over the past decade.

Among former members of HTS and allied groups, Moustafa, said, “It’s a fact that even those who carry the most fundamentalist of beliefs, the most conservative within the fighters, have a vehement hatred of ISIS.”

“The coalition between the United States and Syria is the most important partnership in the global fight against ISIS because only Syria has the expertise and experience to deal with this,” he said.

Later Sunday, Syria’s state-run news agency SANA reported that four members of the internal security forces were killed and a fifth was wounded after gunmen opened fire on them in the city of Maarat al-Numan in Idlib province.

It was not immediately clear who the gunmen were or whether the attack was linked to the Saturday’s shooting.