President Obama said Wednesday he was "saddened and outraged" by the shooting deaths of two U.S. Air Force personnel in Frankfurt Germany, adding that his administration would "spare no effort" in its investigation of the incident.

In a surprise appearance at the White House press briefing, Obama called the attack a tragedy and said the U.S. will ensure the perpetrators are brought to justice. It happened on a bus outside the Frankfurt airport on Wednesday.

The incident is a "stark reminder of the extraordinary sacrifices that our men and women are making all around the world to keep us safe and the dangers that they face all around the globe," he said.

The U.S. has reduced its forces in Germany over the last decade, but still has some 50,000 troops stationed there. They have been targeted in attacks in the past. The Air Force said the personnel killed and wounded Wednesday were based in Britain, where the 48th Fighter Wing is stationed in Lakenheath.

The German news agency DAPD quoted a local official as saying there were no indications of a terrorist attack.

The Associated Press contributed to this report.