The father of missing university chef Claudia Lawrence said he hoped the new forensic search would help solve the mystery of his daughter's disappearance four-and-a-half years ago.
Peter Lawrence said: "We obviously hope because we need to know what happened to Claudia."
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The violence began after a counter-protester allegedly doused a female rally organiser with pepper spray.
As temperatures climb through the day, the warmest spots are likely to be across western parts of Northern Ireland and western Scotland.
The foreign secretary has criticised internet and social media giants for failing to detect and remove extremist material posted online.