The European Space Agency and launch company Arianespace say the satellites ended up in off-target orbits after being launched Friday from Kourou, French Guiana, aboard a Soyuz rocket.
Saturday's agency statement did not explain whether their orbital paths could be corrected. Arianespace said the satellites settled into a lower, elliptical orbit instead of the circular one intended, and initial analyses suggested the mishap occurred during the flight phase and involved the Fregat upper stage of Soyuz.
"Our aim is of course to fully understand this anomaly," Stephane Israel, Arianespace chairman and CEO, said in a statement. "While it is too early to determine the exact causes, we would like to offer our sincere excuses to ESA and the European Commission for this orbital injection that did not meet expectations."
Israel said Arianespace along with customer ESA and the Commission will create an independent panel to investigate what caused the inaccurate deployment and to develop corrective actions so Soyuz launches can resume.
The European Union hopes to have its 30-satellite Galileo navigation network operating fully by 2020. The Prague-based program oversaw the launch of its first two satellites in 2011, two more in 2012, and two more Friday.
Jean-Yves Le Gall, president of the French space agency CNES, said the investigation still needed to determine precisely how far off course the satellites were. He said European Space Agency experts in Toulouse, France, and Darmstadt, Germany, were calculating whether small motors inside the satellites would be strong enough to push them into the correct orbit.
Le Gall told The Associated Press in a telephone interview that the investigation would take "several days to understand what has happened. And then we'll see about the possible consequences on the launch calendar," he said, referring to plans to launch more satellites in coming months.
He called the Galileo navigation network "a very complex program, and even if we have some failures, that's unfortunately part of the life of operations."
If the two satellites cannot be pushed to the correct altitude above the earth, he said, subsequent satellites launched would have to take up the slack.
The program has faced other delays and operational hiccups. European Space Agency officials said Wednesday they had to reduce the strength of another Galileo satellite's signal because of unspecified problems.
The European agency says it hopes Galileo will provide greater precision for satellite navigation systems than the GPS system already used worldwide to pinpoint locations and plot routes.