A total of 7.9 million immigrants have come to the USA since 2000, making the first half of this decade the highest period of immigration in U.S. history despite tougher scrutiny after 9/11, figures released Monday show.

Almost half, or 3.7 million, entered illegally, according to an analysis of Census data by the Center for Immigration Studies, a Washington, D.C., group that advocates controlling the flow of legal and illegal immigrants.

The report comes as a heated immigration debate is likely to take place this week in Congress. The House is expected to tackle a Republican bill that aims to strengthen border security and increase penalties for illegal immigration. An estimated 11 million immigrants live illegally in the USA.

The nation's immigrant population hit a record 35.2 million in March 2005, 2 1/2 times the number at the peak of the last great immigration wave of 1910, says Steven Camarota, author of the report. Immigrants make up 12.1% of the U.S. population, compared with 14.7% in 1910.

Despite an uptick in the educational levels of immigrants in recent years, the analysis shows that 31% of adult immigrants have not completed high school. A third lack health insurance.