Russian former double agent Sergei Skripal and his daughter Yulia remain in a critical condition in intensive care in hospital, British police said on Wednesday as they appealed for witnesses to come forward with any information about the case, Reuters reported.

Skripal, 66, and Yulia, 33, were found unconscious on Sunday on a bench outside a shopping center in the southern English city of Salisbury after being exposed to an unknown substance.

Police said they were keen to speak to anyone who visited two venues in the city where Skripal and his daughter were thought to have been, the Zizzi pizza restaurant and the Bishop’s Mill pub.

“The focus at this time is to establish what has caused these people to become critically ill,” said Mark Rowley, head of London’s counter-terrorism police who are in charge of the investigation.

Police cordons remained in place in several locations in Salisbury, with new cordons added near Solstice Park in the nearby town of Amesbury.