This handout photo provided by the US Army, taken Oct. 26, 2013, shows a 10th Combat Aviation Brigade CH-47 Chinook helicopter, operated by members of the Texas and Oklahoma National Guard, flying a personnel and equipment movement mission over Kabul, Afghanistan. To outfit Afghanistan’s security forces with new helicopters, the Pentagon bypassed U.S. companies and turned instead to Moscow for dozens of Russian Mi-17 rotorcraft at a cost of more than $1 billion. Senior Pentagon officials assured skeptical members of Congress they’d made the right call, pointing repeatedly to a top-secret 2010 study they said named the Mi-17 as the superior choice. (AP Photo/Capt. Peter Smedberg, USCArmy)

Some answers to questions about how the United States ended up buying more than $1 billion in new helicopters from the Russian arms export agency that has supplied weapons to Syria's military.

—Why Russia? Defense Department officials have maintained that Afghanistan's national security forces need heavy-duty helicopters capable of moving troops and supplies in rugged conditions, and that Russia's Mi-17 is best suited for the mission. The Afghans have years of experience flying the Russian chopper.

—Why couldn't an American-made helicopter do the job? Turns out one actually could. A top-secret Pentagon study found that the Chinook, a heavy-lift helicopter built by Boeing in Pennsylvania, was the most cost-effective option for the Afghans. Congressional critics of the Mi-17 contract said the Pentagon used the study to prove the necessity of buying Mi-17s and never mentioned the Chinook. The Pentagon declined to speak on the record about the study or provide any details about its conclusions.

—What is Rosoboronexport? That's the Russian arms export agency the U.S. signed a contract with a few years ago for the Mi-17s. Doing business with Rosoboronexport before then was forbidden. The Bush White House had imposed penalties against the agency in 2006 after it determined Rosoboronexport had provided sensitive military technology to Iran and Syria. The Obama administration lifted the sanctions in 2010, one of a number of diplomatic moves aimed at "resetting" relations between the former Cold War adversaries.

—How many members of Congress are opposed to buying Mi-17s? A lot. From both political parties, too. In July, for example, more than 80 House members voiced their opposition to the contract with Rosoboronexport in a letter to Defense Secretary Chuck Hagel.