Edwin Lyman, a senior staff scientist with the Union of Concerned Scientists, which opposes nuclear power, said the case raises questions about security at the nation's nuclear power plants -- even though Mobley has not been linked to any wrongdoing at any plants.
Some of the information used to give temporary workers like Mobley clearance comes from other nuclear power companies and is sometimes incomplete, Lyman said.
"The real question is: Was there information that the NRC or utilities could have seen that would have led to his disqualification?" Lyman asked.
The Nuclear Regulatory Commission said Friday that Mobley worked between 2002 and 2008 for contractors who did work at the Salem and Hope Creek plants in New Jersey; the Peach Bottom, Limerick and Three Mile Island facilities in Pennsylvania; and Calvert Cliffs in Maryland.
Officials at PSEG Nuclear, which runs the complex in New Jersey, say he carried supplies and worked on routine maintenance mostly during periodic refueling outages, when hundreds of contracted employees descend upon the plants.
The NRC says a laborer typically would not have access to security-related or sensitive information.
Officials also say he passed screenings before he could work at the plants. The NRC says the screenings include criminal history checks, drug testing, psychological assessments and identity verification. The background checks are to be performed by either the nuclear plant operators or their contracting companies.
The plants also run behavior observation programs in which employees are taught to recognize and report suspicious activities.
Mike Drewniak, a spokesman for New Jersey Gov. Chris Christie, said Mobley was never reported to be acting improperly and was not believed to have been involved in any breaches at the New Jersey plants.
Mobley is a 26-year-old natural-born U.S. citizen who grew up in Buena, N.J., and later lived in Philadelphia and Newark, Del. A former neighbor said he moved to Yemen about two years ago, supposedly to learn Arabic and study Islam.
He was among 11 al-Qaida suspects detained this month in a security sweep in Yemen's capital of San'a this month. He was taken to the hospital over the weekend after he complained of feeling ill. Yemeni officials said he snatched a gun from a security guard and fatally shot one guard and wounded another before being captured.
His parents have said he is not a terrorist.
A former friend said he believed Mobley was becoming radical before he moved to Yemen about two years ago.
Roman Castro, an Army veteran who did a tour in Iraq after he and Mobley graduated from high school together in 2002, said Mobley had only these words for him in a chance meeting four years ago: "Get the hell away from me, you Muslim killer!"