A third child was wounded and remains in a critical condition in hospital.
Police Lt. Matt Murray says it appears to have been an isolated shooting and that the killer is not thought to be at large.
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The letter was signed by the CBI, British Chambers of Commerce, the EEF, the Institute of Directors and the Federation of Small Businesses.
A spokesperson said Rory Stewart's remarks were in line with the British government’s stated position.
The minister's intervention will come a week after Trump decertified the landmark agreement claiming it was not in the US national interest.