The Obama administration said Sunday that it has enlisted additional computer experts from across the government and from private companies to help rewrite computer code and make other improvements to the online health insurance marketplace, which has been plagued by technical defects that have stymied many consumers since it opened nearly three weeks ago.

This expanded team has come up with new ways of monitoring which parts of the federal Web site,, are having problems and has been taking the site offline for rigorous overnight tests, according to a Department of Health and Human Services spokesman.