Saudi Arabia has started a study.

It wants to know why its air defenses did not stop an attack by the Houthis on Riyadh airport.

The attack killed at least 13 people.

It hurt more than 300 people.

Two regional officials said this on Monday.

The officials did not give their names.

They were not allowed to speak publicly.

The first study showed the Houthis first fired many drones.

The drones cleared the way for cruise missiles and ballistic missiles.

Then those missiles reached King Khalid International Airport.

The Houthis have ballistic missiles.

Their warheads can change direction in flight.

This is hard for Saudi Patriot interceptors.

One official said the study is also looking at why air defenses gave no warning earlier.

The missiles and drones flew over 1,000 kilometers (620 miles) to reach Riyadh.

This attack is one of several Houthi attacks on Saudi airports since last week.

It has shown problems in Saudi Arabia's defense system.

The Saudi-led coalition said it stopped and destroyed ballistic missiles fired toward Riyadh on Monday.

It did this twice.

At least three explosions were heard in the city Monday night.

Later, Civil Defense officials said the danger had passed.

The Saudis want the U.S. to take a more direct role in fighting the Houthis.

The U.S. has shared intelligence.

It has also helped with target planning.

But it has not carried out strikes.

President Donald Trump said Saturday he was thinking about U.S. attacks on the Houthis.