An explosion in Liverpool, England, on Sunday may have been a terrorist attack.

A taxi exploded outside of Liverpool Women’s Hospital, killing a passenger and injuring the driver. The regional counterterrorism unit responsible for the northern English city has taken the lead in the investigation. Three men in their 20s have been arrested thus far, and armed police (police are not traditionally armed in the U.K.) have established cordons on two city streets. Later on Sunday, a reporter for the Liverpool Echo newspaper noted the arrival of police negotiators at one scene, suggesting that a standoff may be underway.