WASHINGTON (CNN) -- The IRS says it's still trying to reach some 5 million Americans -- primarily seniors and veterans -- who have not filed the tax returns needed to qualify for their stimulus checks.
The IRS is trying to reach people who do not normally file tax returns so they can apply for stimulus checks.
"This hasn't been the easiest undertaking for the IRS," Commissioner Doug Shulman told the House Ways and Means Subcommittee on Oversight on Thursday.
Shulman explained that the IRS had sent out information packages to 20.5 million recipients of Social Security or veterans benefits who did not file 2006 tax returns to explain how to claim their payments.
Around 15 million responded.
The commissioner says the agency will soon send a second notice to the remaining 5.2 million and launch an aggressive public relations campaign titled, "It's Not Too Late to File!"
In all, the IRS has sent out more than 76 million stimulus checks, in amounts up to $600 for individuals, $1,200 for married couples and $300 per child for families.
The checks mailed nationwide so far total $63.8 billion, Shulman said. He predicted the total by the end of the calendar year would reach $100 billion.
IRS phone banks have been inundated with questions about the stimulus payments, which began going out in April.
The commissioner said the IRS received 72 million calls in May this year, compared with 19 million in May 2007.
"The numbers are staggering," he said.
Shulman apologized for the now 13-minute average wait time to get an IRS employee on the phone, saying, "It's where we'd not like to be, but it's a fact of doing stimulus."
Nina Olson, the IRS' national taxpayer advocate, said only a few glitches had developed in the stimulus effort. One occurred when the Social Security numbers of approximately 1,500 taxpayers were inadvertently disclosed when the IRS routed the stimulus payments to the wrong bank accounts.
Olson also blamed the lack of participation of more than 5 million seniors and veterans on their lack of access to and experience with computers and the Internet, as well as a belief by some that the filing process is too cumbersome and complicated.
"I think there are some natural barriers to getting some folks to want to file a return again," she explained.
To receive a stimulus check, people had to file a 2007 return. The notices sent to those who didn't file an income tax return in 2006 told them they should file a 1040A tax form for 2007 even if they had no tax liability.