An Uzbek man who pleaded guilty to plotting to assassinate President Barack Obama was never admitted to Columbia University's medical school, the university said Saturday, contrary to defense claims before he was sentenced to more than 15 years in prison.

The disparity could be a misunderstanding linked to language problems, a defense attorney said, but it also could raise questions about Ulugbek Kodirov's credibility and the information he is giving to investigators of terror threats against the United States.

A federal judge sentenced Kodirov, 22, on Friday for plotting to kill Obama. Kodirov could have gotten a sentence twice as long, but U.S. District Judge Abdul K. Kallon agreed to prosecutors' request for a shorter term because the man has provided substantial assistance that the government believes to be truthful.

Prosecutors would not disclose what sort of information Kodirov has provided to investigators, but evidence showed Kodirov believed he was communicating with an Islamic group in his native Uzbekistan that the United States classifies as terror organization.