A police officer in northern India was killed trying to calm a mob angered by reports that cows, regarded by Hindus as holy, had been slaughtered in the area.

BBC Hindi's Nitin Srivastava reports from Bulandshahr in the state of Uttar Pradesh, where the incident happened.

It was sometime after 10:00 local time (04:30 GMT) on Monday when Inspector Subodh Kumar Singh sat in his official vehicle and asked the driver to make the SUV "fly". The driver, Ram Asrey, drove as fast as he could.

Mr Singh, 47, was in charge of the police station in Siyana, a village in Bulandshahr. He had received a call that morning asking him to go to a police station in another village, Chigrawati, about 5km (3 miles) away, where an angry mob of villagers had gathered. They alleged that they had found carcasses of cows earlier that day in the fields - videos, which soon began circulating on WhatsApp and social media, show a tractor with several carcasses blocking the road leading to the police station.

Cows, which are considered holy by India's majority Hindu population, have become a flashpoint in India in recent years.

Many states, including Uttar Pradesh, have started enforcing bans on cow slaughter since the Hindu nationalist Bharatiya Janata Party came to power in 2014.

So-called cow vigilantism has been on the rise and has led to several killings in the past few years. But an attack on police is unheard of.

An 18-year-old protester was also killed in the violence. Police have arrested four people over Inspector Singh's killing and are looking for 23 others.

The mob in Chigrawati, according to eyewitnesses and police officers, quickly grew to include more than 300 people as those from nearby villages also joined in. They were demanding the police take immediate action to find out who was responsible for the alleged slaughter of cows.

And the six officers inside the police station had started frantically calling the district headquarters. It's unclear if they called Mr Singh directly or if he was ordered to go there as part of reinforcements.

When Mr Singh arrived, he approached the mob, hoping to pacify them. Eyewitnesses told the BBC that, unlike some of the other policemen, Mr Singh was not wearing a bulletproof jacket and did not have his pistol in his hand.

The police wanted to calm the villagers down because thousands of Muslims had gathered over the weekend in Bulandshahr for a religious event. So officials feared the mob's anger could spiral and lead to religious rioting.

But, eyewitnesses say, the villagers only got angrier and more aggressive even as more police officers began to arrive.

It's unclear when the violence broke out but one eyewitness, a man who works in a school near the police station, told the BBC that he could hear the two sides fighting for more than a half hour and gunshots rang out at regular intervals. He says he locked himself in the school's bathroom.

Police told the BBC that some members of the mob were armed with homemade revolvers and opened fire first. But eyewitnesses, some of whom were watching from the other side of the road, said the violence broke out only after the police fired into the air to try and disperse the crowd.

The mob also set fire to the police station and several vehicles parked outside. As the officers scurried for cover, the men began to hurl stones and even bricks at them, Mr Asrey said in an interview on Monday.

Some officers managed to flee inside and hide in the police station, but Mr Asrey and another officer ran to the SUV to escape.

"That's when I realised Mr Singh had been hit by a brick and was lying unconscious," he says. So, he put him in the car, got into the driver's seat and started driving away from the mob, which had blocked the road, and towards fields.

Mr Asrey says the mob followed them and tried attacking the vehicle.

"The wheels of the vehicle got stuck. We had no choice but to get out and run."

Mr Asrey was injured but survived. He doesn't know how Mr Singh died.

But a video, which has gone viral since Monday afternoon, shows Mr Singh slumped out of the SUV and motionless. Men could be heard in the video asking whether he was dead or alive while gunshots kept going off in the background.

The inspector, who has a wife and two sons living in Noida near Delhi, was declared dead when he was brought to the hospital.

The post-mortem report says that he suffered a bullet wound above his left eyebrow. A police officer told the BBC that Mr Singh's three mobile phones and pistol were missing.