Officials warned the country’s 3.5 million citizens to be on the lookout for a missing radioactive United States $100 bill. They say they do not know how the bill became radioactive. One theory is that it could have come from somewhere in Eastern Europe that still has a high level of post-Chernobyl contamination. The bill was discovered in September when it set off alarms at an airport checkpoint and was quarantined in a safe room. But it was reported missing — probably stolen — on Tuesday just before it was to be shipped off to be destroyed. A security official said the bill was not potent enough to cause burns, but could pose risks for children or pregnant women. In Lithuania, $100 is equivalent to about two-thirds of the monthly minimum wage.