Engineers at Nasa headquarters are viewing radar launch imagery and photographs taken by the crew of the international space station before the shuttle docked.
According to John Shannon, chairman of the mission management team, a piece of foam the size of a grapefruit come off a bracket on the fuel tank, then bounced off a strut further down and shot into Endeavour.
The brackets hold the long fuel feed line to the tank, and the struts connect the tank to the shuttle for launch.
Ice forms near these brackets and causes the foam to pop off at lift-off.
Shannon said that the foam had come loose on previous flights, and Nasa is looking at how to redesign the apparatus.
He said: "It's a little bit of a concern to us because this seems to be something that has happened frequently."
In the 26 years of shuttle flight, almost every mission has ended with dents and scratches of at least an inch in the thermal tiles that cover the belly.
In one flight, nearly 300 dents were recorded.
The ship's belly is exposed to temperatures as high as 2,300 degrees Fahrenheit while passing through Earth's atmosphere.
Shuttle wings encounter even more heat, which is why the briefcase-sized foam that hit the space shuttle Columbia's left wing at lift-off resulted in the shuttle exploding in 2003 over Texas during re-entry.
Since that disaster, Nasa has equipped its crew with tile-repair kits.
Depending on the extent of the damage, astronauts can apply protective paint or insert a shielding panel.
These repairs could be made during a fourth spacewalk next week, but managers have said they are less likely to resort to that option now that they know the damage was caused by foam.
Nasa wants to keep Endeavour at the space station for at least seven days.