Saudi Arabia has opened an investigation into its air defenses.

The question is why they failed to stop a Houthi attack on Riyadh airport.

The strike killed at least 13 people and wounded more than 300.

Two regional officials said this on Monday.

The officials spoke on condition of anonymity.

They were not authorized to comment publicly.

The early probe showed the Houthis first fired many drones.

These cleared the way for cruise and ballistic missiles to reach King Khalid International Airport.

The Houthis have ballistic missiles with warheads that can change direction in flight.

This is a problem for Saudi Patriot interceptors.

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

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

The attack is one of several Houthi strikes on Saudi airports since last week.

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

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

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

Civil Defense officials later 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 and helped with target planning.

But it has not carried out strikes.

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