Police searching for two missing children believed to have been taken by their father now say they fear for their lives because there are “indications” he planned to kill them and then himself.

On Monday afternoon police found tyre tracks leading from a grass field into a canal but a search of the area has failed, so far, to find the car or any trace of the children.

Earlier in the afternoon they said they had found possible evidence during their search of a rural location near Finsterwolde and had sealed off the area for further investigation.

Jeffrey (10) and Emma (8) have been missing since Saturday afternoon and are thought to have been taken by their father Klaas Bijl, who is 67. An amber alert was issued on Sunday amid growing concerns for the children’s safety.

The search of the Carel Coenraad polder began on Sunday morning following several tip-offs. On Monday morning, police said they had found no trace of the children, but later said they had discovered “signs” and were working to establish whether these are linked to their disappearance.

The hunt began after family members raised the alarm. A half-brother said on social media that the children had been seen with their father in an amusement arcade just across the border in Germany. He was reportedly drunk and had threatened to harm himself and the children, the relative is quoted as saying.