CINCINNATI (REUTERS) - A US judge ruled on Friday that an Ohio man charged with plotting to attack the US Capitol with guns and bombs be held without bail after prosecutors said he posed a threat to national security.

Christopher Cornell, 20, of Cincinnati, was arrested last week.

He researched the construction of pipe bombs, purchased a semi-automatic rifle and 600 rounds of ammunition and made plans to travel to Washington to carry out the plot, according to an FBI informant's legal testimony.

US prosecutors said Cornell should remain behind bars until his trial because of the serious nature of the plot.

US Magistrate Judge Stephanie Bowman granted their request to deny bail in a hearing that lasted less than 15 minutes.

Lawyers for Cornell were seeking his release on bond saying he had not been in trouble before, did not have a passport and did not pose a flight risk.

They proposed that he be sent to the home of his parents and electronically monitored.

Court documents showed that Cornell indicated on Twitter that he supported the Islamic State group under the alias Raheel Mahrus Ubaydah.

According to the documents, Cornell indicated to an FBI informant that "he considered the members of Congress as enemies and that he intended to conduct an attack on the US Capitol in Washington, DC".

Cornell has been charged in a federal court in Ohio with attempting to kill a US government officer and possession of a firearm in furtherance of an attempted crime of violence.