A chunk of stimulus payments are missing in action, thanks to a mix up that put as many as 13 million checks into invalid bank accounts.Why it matters: The IRS (by law) was supposed to get all payments out by Friday. Now the onus could shift to Americans to claim the money on their tax refund — further delaying relief to struggling, lower-income Americans.Get smarter, faster with the news CEOs, entrepreneurs and top politicians read. Sign up for Axios Newsletters here.What's going on: The newest COVID-19 relief bill — signed in the final days of 2020 — mandated the $600 payment to those making up to $75,000 per year (or 150,000 for joint filers) get out by Jan. 15. * The fast turnaround meant “some payments may have been sent to an account that may be closed or, is or no longer active, or unfamiliar,” according to the IRS website.To get a sense of the speed: It took 19 days to distribute half the first-round payments last spring, but two-thirds of payments were out the door just a week after the latest bill became law, according to an analysis by the Committee for a Responsible Federal Budget. * Billions of those dollars are in the process of being returned to the IRS by tax preparers because of the error, though the IRS would not say how many payments were incorrectly deposited. * Jackson Hewitt estimates funds were deposited in 13 million accounts that were no longer open.How it works: These accounts are typically set up by tax prep companies, most often used by financially constrained taxpayers to get their refunds faster. * Some tax preparers told CNBC that the money would be deposited starting Feb. 1. What’s next: It’s up to those whose payments haven’t been disbursed by today to claim what’s owed on their tax return. * “You can wait until the money shows up, or you’re going to file your return and claim your money there,” Janet Holtzblatt, a senior fellow at the Tax Policy Center and former official at the Treasury Department’s Office of Tax Analysis, tells Axios. * “There’s going to be confusion” about which option to pick.Of note: Any refunds that also claim the earned-income tax credit — which offsets tax bills for lower income workers — can’t be issued before mid-February, prolonging the delay as the Washington Post points out.What to watch: The incoming Biden administration wants to issue another round of direct payments. Depending on the timing, the IRS could be juggling those checks at the height of tax season. * “I can never say with IRS that things are impossible, but it's going to be a challenge to get those payments out during filing season,” Holtzblatt says.You can check the status of your stimulus payment — and whether you can expect it by paper check, debit card or direct deposit — here.Be smart: sign up FREE for the most influential newsletter in America.