Here’s the answer offered by senior U.S. officials: The Iranians are stressed, at home and abroad, in ways that are leading them to engage in riskier behavior.

Officials say Quds Force operations have been more aggressive in several theaters: in Syria, where the Iranian operatives are working covertly to help protect the embattled regime of President Bashar al-Assad; in Iraq, where the Quds Force this year stepped up attacks against departing U.S. forces; in Afghanistan, where they have been arming the Taliban; in Azerbaijan, where they have been more aggressive in projecting Iranian influence; and in Bahrain, where their operatives worked to support and manipulate last spring’s uprising against the Khalifa government. (Shakuri, who was indicted Tuesday, is said to have helped plan Quds Force operations in Bahrain.)

But why the use of Mexican drug cartels? U.S. officials say that isn’t as implausible as it sounds. The Iranians don’t have the infrastructure to operate smoothly in the United States. They would want to use proxies, and ones that would give them “deniability.”

“They’re very willing to use all kinds of proxies to achieve specific clandestine foreign-policy goals,” says a senior U.S. official who has been briefed on intelligence reports.