Warsaw, Poland (CNN) President Barack Obama said Friday that the nation was "horrified" after what he called "a vicious, calculated and despicable attack on law enforcement" in Dallas.

The White House announced Friday evening that the President would cut short his European trip by one day and visit Dallas early next week.

It was the second somber statement about shootings on American soil the President made from Europe in less than 12 hours. He condemned the recent shootings by police of black men in Minnesota and Louisiana shortly after arriving in Europe and made a broad diagnosis of American racial imbalance. Hours later, after chaos erupted in Dallas and five police officers were killed in sniper shootings that began during protests stemming from the Minnesota and Louisiana shootings, he promised justice.

"There is no possible justification for these kinds of attacks or any violence against law enforcement," Obama said, speaking from the NATO summit in Poland that he is attending. "Anyone involved in the senseless murders will be held fully accountable. Justice will be done."

Obama also ordered flags lowered to half staff at the White House and on public buildings and grounds.

His overseas trip comes at a time when the country has been shaken by violence, during a week with fatal clashes between police and communities of color, ending with the shootings in Dallas, the single deadliest day for law enforcement since September 11, 2001.

The events also come during a heated presidential election where issues of gun violence and criminal justice reform have been in the spotlight, and come just hours before presumptive Democratic presidential nominee Hillary Clinton was scheduled to give a speech addressing the shootings from earlier in the week.

"I mourn for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them. -H" Clinton tweeted

I mourn for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them. -H — Hillary Clinton (@HillaryClinton) July 8, 2016

Spilling over to the campaign trail, presumptive Republican presidential nominee Donald Trump posted his condolences to his Twitter feed Friday morning.

"Prayers and condolences to all of the families who are so thoroughly devastated by the horrors we are all watching take place in our country," the real estate mogul wrote , a somber and measured statement from a candidate who has been an outspoken advocate for law enforcement throughout his campaign. He later sent a longer statement citing, "senseless, tragic deaths of two motorists in Louisiana and Minnesota," a reference to the two black men killed earlier this week at the hands of law enforcement.

Prayers and condolences to all of the families who are so thoroughly devastated by the horrors we are all watching take place in our country — Donald J. Trump (@realDonaldTrump) July 8, 2016

Others political figures blamed Obama for what they saw as rhetoric that led to police violence. Former congressman Joe Walsh tweeted and then deleted a message attributing the President's words and the Black Lives Matter movement for inciting violence.

"3 Dallas Cops killed, 7 wounded. This is now war. Watch out Obama. Watch out black lives matter punks. Real America is coming after you," Walsh, who's now a talk radio host, tweeted , later clarifying that he did not intend to call for violence.

Even during the high-stakes summit focused on European unity, issues of race and violence dominated Obama's early agenda in Poland. Obama, who noted he's been receiving regular updates on the Dallas shootings, said he'd conveyed condolences to the city's mayor, Mike Rawlings, and indicated the FBI was involved in investigating the shootings, which left five police officers dead.

Obama said he would weigh in again when more is known about the killers' "twisted motivations."

The President also made an implicit call for tighter gun restrictions, saying the carnage could have been mitigated if the killer or killers weren't carrying powerful weapons.

"Today is a wrenching reminder of the sacrifices they make for us," Obama said of law enforcement officers. "We also know when people are armed with powerful weapons, unfortunately, it makes attacks like these more deadly and more tragic."

Obama said he wanted Friday to "focus is on the victims and their families."

"They are heartbroken," Obama said. "The entire city of Dallas feels this to its core. We are grieving with them. I ask you say a prayer for the officers and their families. Keep them in your thoughts."

Obama was speaking from Warsaw, where he arrived early Friday for a summit of NATO leaders. His remarks on Dallas came at the end of a session with the presidents of the European Council and European Commission, where Obama plotted a path forward after Britain's vote to exit the EU.

After arriving in Poland in the early morning hours Friday, Obama went to his hotel and delivered a statement about two police-involved shootings this week, saying the episodes reflected persistent racial disparity.

Citing statistics showing minorities are more likely to be pulled over, searched or shot at by police, Obama said it was incumbent on the country to aim for better -- including recognizing deep-seated biases that he said must be "rooted out."

"If communities are mistrustful of the police, that makes those law enforcement offers who are doing a great job, who are doing the right thing, that makes their lives harder," Obama said, insisting that recognizing problems within law enforcement doesn't equate to being anti-police.

"When people say 'black lives matter,' it doesn't mean that blue lives don't matter," Obama said, referring to police officers. "But right now, the data shows that black folks are more vulnerable to these kinds of incidents. There is a particular burden that is being placed on a group of our fellow citizens."