There was a deadly car explosion in Liverpool, a city in Northwestern England, over the weekend. On Monday, the police declared it as a “terrorist incident.” The blast took place outside a busy women’s hospital. The driver had a lucky escape, while a male passenger in the car was declared dead on the scene. Both have not as yet been named.

The suspected attacker died as a homemade bomb exploded in a taxi, outside the Liverpool Women’s Hospital on Sunday, Remembrance Day, according to a BBC News report. It named the injured driver as David Perry. BBC security correspondent Gordon Corera said that the passenger was not believed to have been known to MI5.

The passenger who is reported to have made the device and brought it to the taxi, died at the scene of the explosion. A explosion occurred at the drop off point at the hospital. The driver escaped from the cab.

At a news conference, Russ Jackson, head of Counterterrorism Policing in northwest England also said that although the motivation had not been understood, the circumstances were such that it was declared to be a “terrorist incident” and that the investigation by the counter terrorism police would be ongoing.

Later on Monday, MI5, the British intelligence agency said that the country’s terrorism “threat level” had been increased to “severe.” A severe threat means that “an attack is highly likely.”

The outlet reported that four men between the ages between the ages of 20 to 29 have been arrested in the city. Three men aged 21, 26 and 29 were detained on Sunday at Sutcliffe Street in Kensington on Sunday while the fourth aged 20 was arrested earlier in the Kensington area and was in custody.

Russ Jackson also said that the four men were believed to be associates of the passenger in the taxi. He said that the police knew the attacker’s identity but did not release a name.

A spokesman for Liverpool Hospital said that patients could attend their appointments as usual. However, visiting arrangements were restricted as of now.