NEW DELHI (AP) – Two bombs exploded at New Delhi’s main mosque shortly after hundreds of worshippers offered Friday prayers, injuring at least 13 people, a top official said.

“Thirteen people, injured in the two explosions, are in hospital,” said Chief Minister Sheila Dikshit, New Delhi’s top elected official.

The two blasts occurred within 30 minutes of each other at the Jama Masjid mosque, said Police Chief K.K. Paul. “We are investigating what kind of explosive devices were used,” Paul said.

The mosque was evacuated and police bomb disposal squads combed the mosque complex, he said. Police went on high alert and stepped up security across the city.

The explosions occurred just before hundreds of people were to gather at the mosque for evening prayers.

It was not known who was behind the explosions, which were caused by at least two bombs planted in different parts of the complex.

The first bomb exploded near the washing area of the mosque, where people clean their hands before offering prayers, said Imam Bukhari, the mosque’s chief cleric.

The 17th century Jama Masjid is located in one of the most congested neighborhoods of New Delhi and is surrounded by hundreds of shops and houses lining a maze of crowded alleyways.

Bukhari appealed to people to remain calm and said evening prayers would take place as usual. “People were walking around and suddenly, boom! there was a bomb,” said Mohammed Salaudi, who was inside the mosque when the first explosion took place. Salaudi said he saw at least three injured people after the explosion in the main courtyard of the complex.

Police began letting people enter the main mosque for evening prayers and hundreds of worshippers were back in the mosque less than two hours afterward.

Most of them stopped near the site of the blasts to stare at the red sandstone floor of the mosque, which had turned bluish black from the blast’s impact.

Half a dozen shoes and flip-flops, some charred from the explosion, lay scattered on the floor, left behind as people fled after the blasts.

“Who would do something like this? What sort of person is this?” asked Mustafa Iqbal, a trader who had come for evening prayers, as he looked at the shoes.