The Internal Revenue Service is struggling to respond to an onslaught of tax returns filed on April 15, which is delaying acknowledgements to both federal and state returns.

“Due to the significant increase in federal and state submissions transmitted on April 15th, the length of time to create federal acknowledgments and make them available for retrieval is taking longer than expected,” the IRS said in an email to tax professionals Tuesday evening. “The IRS is closely monitoring the acknowledgment rates and is working to close the gap as a top priority.”

The IRS added that the majority of state tax returns are linked to the acceptance of the federal return, so the length of time it takes to make the state return available for state retrieval has also increased. It asked tax professionals not to re-transmit any tax returns if they have not yet received an acknowledgment.

“As the federal backlog decreases, the state submissions will then be ready for state pickup,” said the IRS. “In the interim, please do not retransmit any submissions awaiting acknowledgment if the IRS has issued a receipt. We thank you again for your patience and support.”

The IRS is working its way through the backlog. Anecdotally, Intuit sent back an acknowledgment Tuesday afternoon that the IRS had accepted a tax return that had been filed close to midnight on April 15 with TurboTax.