Police in the Indian capital territory of Delhi have invoked an “anti-terror” law as they probe a deadly car explosion near the historic Red Fort on Monday, in which at least nine people were killed, local media reported.

Indian television channels reported that at least 20 people were also wounded in the blast.

Indian Minister of Home Affairs Amit Shah, who visited the site of the blast as well as Lok Nayak Hospital where the dead and injured were taken, promised an in-depth probe into the incident.

“We are exploring all possibilities and will conduct a thorough investigation, taking all possibilities into account. All options will be investigated immediately, and we will present the results to the public,” Shah said.

But by Tuesday morning, police appeared to be focusing on the possibility that the explosion was an intentional act, rather than an accident. Multiple Indian news agencies reported that the Delhi Police had registered a case under the Unlawful Activities Prevention Act (UAPA), the country’s main law to combat acts of alleged terrorism, and the Explosives Act, which punishes the illegal purchase and use of incendiary materials.

Officially, though, law enforcement agencies had not pinpointed a cause for the explosion, which occurred near the underground metro station at the popular tourist destination, also known as the Lal Quila.

The neighbouring Uttar Pradesh region and India’s financial capital, Mumbai, were placed on high security alert in the wake of the incident, Indian media reported. So too were nearby sites, including Indira Gandhi International airport, according to a statement by India’s central police force.

In videos shared online, the wreckage of several cars could be seen on a congested street in the old quarter of Delhi as police poured into the area to secure it and push back gathering crowds.

The explosion caused six vehicles and three autorickshaws to catch fire, according to New Delhi’s deputy fire chief.

Delhi Police Commissioner Satish Golcha said the blast happened shortly before 7pm (13:30 GMT).

“A slow-moving vehicle stopped at a red light. An explosion happened in that vehicle, and due to the explosion, nearby vehicles were also damaged,” he told reporters, adding that the situation was being monitored.

Speaking to Al Jazeera, Nazish Malik said his brother-in-law, Mohsin, an e-rickshaw driver, was missing after the blast.

“After more than a hundred calls on his phone, finally someone answered and said that the phone is at the police post at the Red Fort and your relative has been taken to Lok Nayak Hospital,” Malik said.

“When we heard that, we rushed here and got here around 8.30pm. My brother was already here. I got here late, but found that the security was so tight, they are not permitting us to go inside,” he added.

“At least announce who is critical and who is dead. You can at least inform that bit,” he said.

Red Fort blast raises security concerns

The 17th-century Mughal-era Red Fort, which was formerly an imperial palace, is a major tourist attraction.

Speaking to Al Jazeera, independent journalist Amit Baruah described the area as “one of the most crowded parts of the city”, something he said made it “vulnerable” to attacks.

“It shatters the calm of the capital city,” said Baruah, who noted that Delhi had not experienced a blast like Monday’s in recent memory.

“This is an incident that will definitely cause concern to ordinary people living not just in that area but around,” he noted.

Indian Prime Minister Narendra Modi offered his sympathies to the families of the victims.

“Condolences to those who have lost their loved ones in the blast in Delhi earlier this evening. May the injured recover at the earliest,” Modi wrote on X.