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President Obama's words - swift, solemn and understated - stressed three main points. The nation's sympathy for Boston. The fact that the motives for the attack were as yet unknown. His determination to catch and punish the person or people responsible.

But what came over more than anything was a frustration that so much is unknown.

Much will be said in the coming days about terrible crimes like this bringing a nation together but they can also divide, and raise questions about leadership.

The truth is that it is difficult for the president to strike the right tone in the very midst of uncertainty. His words, hours after the attack, will have to bear scrutiny in the days, weeks and years to come. The wrong implication or interpretation could come back to haunt him.

He - apparently very deliberately - did not use the word terrorism even though he has been criticised in the past for not being quick enough to use the label.

Indeed he has already been criticised for not using it now, but apparently feels caution and certainty are more important than the barbs of critics - particularly when, to many Americans, the word terrorism is misunderstood to only mean action by foreigners.

Nevertheless, a White House official was quick to stress after the statement that this was being treated as an act of terrorism.

Indeed it does seem fairly obvious that it was an attack deliberately planned to cause death and injury. In most people's book that is terrorism. But what if the motive wasn't political, but some other grievance by an individual? This president can be careful with words, and likes to be certain of his facts before making judgements. Some find that irritating. Others just want to make political capital out of any situation.

There will be other questions - about whether intelligence services missed anything, whether security should be higher around the nation, and many more questions that may not yet be obvious.

President Obama will have to balance the firmness and resolution the country expects with his clear desire not to be pushed into snap solutions ahead of clear answers.