In 2001, the for-profit University of Phoenix awarded 72 education degrees through its online program; last year, it awarded nearly 6,000 education degrees—more than any other university.

Virtually unknown a decade ago, big online teacher education programs now dwarf their traditional competitors, outstripping even the largest state university teachers’ colleges.

A USA TODAY analysis of newly released U.S. Department of Education data finds that four big universities, operating mostly online, have quickly become the largest education schools in the U.S. Last year the four schools—three of which are for-profit—awarded one in 16 bachelor’s degrees and post-graduate awards and nearly one in 11 advanced education awards, including master’s degrees and doctorates.

A decade ago, in 2001, the for-profit University of Phoenix awarded 72 education degrees to teachers, administrators, and other school personnel through its online program, according to federal data. Last year, it awarded nearly 6,000 degrees, more than any other university. By contrast, Arizona State University, one of the largest traditional education schools in the U.S., awarded 2,075 degrees, most of them on campus. Columbia University’s Teachers College awarded 1,345 degrees.

Traditional colleges still produce most of the bachelor’s degrees in teaching—ASU topped the list with 979 bachelor’s degrees in 2011. But online schools such as Phoenix and Walden University awarded thousands more master’s degrees than even the top traditional schools, all of which are pushing to offer online coursework. Every one of the top 10 now offers an online education credential.

“We shouldn’t be surprised, because the whole industry is moving in that direction,” said Robert Pianta, dean of the University of Virginia’s Curry School of Education. “The thing I would be interested in knowing is the degree to which they are simply pushing these things out in order to generate dollars or whether there’s some real innovation in there.”

For-profit universities have been the subject of intense scrutiny in Congress.

Sen. Tom Harkin, D-Iowa, who chairs the Senate Health, Education, Labor, and Pensions Committee, last week released findings from a two-year investigation showing that they cost more than comparable not-for-profit schools and have higher dropout rates. For-profits, the investigation found, enroll about 10 percent of U.S. college students but account for nearly 50 percent of student loan defaults.

Online education schools, many of which have open-enrollment policies similar to community colleges, say their offerings are high quality. Most of the top ones are accredited by the same organizations that certify traditional teacher education programs. And they stress that students don’t just sit around in their pajamas.