(usat) The tax system collects its due, even from a class of workers with little likelihood of claiming a refund and no hope of drawing a Social Security check. Illegal immigrants are paying taxes to Uncle Sam, experts agree. Just how much they pay is hard to determine because the federal government doesn't fully tally it. But the latest figures available indicate it will amount to billions of dollars in federal income, Social Security and Medicare taxes this year. One rough estimate puts the amount of Social Security taxes alone at around billion per year. Paycheck withholding collects much of the federal tax from illegal workers, just as it does for legal workers. The Internal Revenue Service doesn't track a worker's immigration status, yet many illegal immigrants fearful of deportation won't risk the government attention that will come from filing a return even if they might qualify for a refund. Economist William Ford of Middle Tennessee State University says there are no firm figures on how many such taxpayers there are. The Social Security Administration estimates that about three-quarters of illegal workers pay taxes that contribute to the overall solvency of Social Security and Medicare. The agency estimates that for 2005, the last year for which figures are available, about billion in taxes was paid on about billion in wages from people who filed W2 forms with incorrect or mismatched data, which would include illegal immigrants who drew paychecks under fake names and Social Security numbers. Spokesman Mark Hinkle says Social Security does not know how much of the billion can be attributed to illegal immigrants. The number is certainly not 100%, but a significant portion probably comes from taxes paid by illegal immigrants. Nine billion dollars sounds like a lot of money, and it is, but it is only about 1.5% of the total 3 billion paid into Social Security in 2005. The impact on Social Security is significant, though, because most of that money is never claimed by the people who pay it but instead helps cover retirement checks to legal workers. Federal law prohibits paying Social Security to illegal immigrants, but the administration factors in both legal and illegal immigration when projecting the trust fund's long-term solvency.