WASHINGTON – American and Afghan officials are expanding the range of explanations for a surge in “insider attacks” on U.S. troops, adding Wednesday the theory that the burden of fasting during the Muslim holy month of Ramadan combined with the summer heat may have prompted more Afghan soldiers and police to turn their guns on their American partners.

Whatever the underlying reasons, the attacks are taking a toll and raising questions about the risk of American and other coalition troops working side by side with Afghan troops as advisers, mentors and trainers. The close contact is an essential element of the U.S. strategy for putting the Afghans in the lead combat role as the U.S. prepares to pull out its last combat troops at the end of 2014.

The top commander of coalition forces in Afghanistan, Marine Gen. John R. Allen, said Thursday that while the reasons for the killings are not fully understood, the effect of Ramadan fasting is likely among the causes.