The Associated Press has reported that the gunman involved in the Santa Monica shooting was related to two of the victims.
BREAKING: Law enforcement officials: Santa Monica gunman related to first 2 victims
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The incident is not believed to be terror-related, British Transport Police said.
The royal couple have been in the US to mark the 300th anniversary of the foundings of New Orleans and San Antonio.
A small number of people were treated at Southgate Tube station, British Transport Police said.