Dallas – A suspect in the Dallas mass shooting that killed five police officers has died after a tense standoff with police in a downtown garage, US media reported, citing law enforcement sources, while US President Barack Obama condemned the attacks from a Nato summit in Warsaw.CNN cited a police source as saying the suspect was killed, though some local media outlets said that the suspect had shot himself.Dallas Police Major Max Geron said officers were conducting "extensive" sweeps across the downtown area of the usually bustling Texas city. The area was on lockdown, with no bus or rail service and flight restrictions.Meanwhile, speaking from Warsaw in Poland, US President Barack Obama on Friday denounced the "despicable" attacks in Dallas."We still don't know all the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement," Obama said during a press conference at a Nato summit in Warsaw, adding that the officers were "targeted" by multiple suspects in the "tremendous tragedy"."Police in Dallas were on duty, doing their jobs keeping people safe during peaceful protests... we are praying for their recovery," he said.'Justice will be done'"According to police, there are multiple suspects. We will learn more undoubtedly about their twisted motivations," Obama continued. "Let's be clear. There is no possible justification for these kinds of attacks or any violence against law enforcement.""Anyone involved in the senseless murders will be held fully accountable. Justice will be done."Obama said the killings once again raised the issue of gun control in the United States after a string of mass shootings. "We also know when people are armed with powerful weapons, unfortunately, it makes attacks like these more deadly and more tragic," he said."In the days ahead, we will have to consider those realities as well. In the meantime, today, our focus is on the victims and their families."Several hundred people attended the Dallas rally, which ended just before the shots rang out around 21:00 on Thursday night (04:00 SA time, Friday).The protest was one of several nationwide over the fatal police shootings of black men in Minnesota and Louisiana this week.