US authorities on Wednesday charged three men with interfering with the investigation of the Boston Marathon bombing, saying they hid fireworks and a backpack belonging to one of the suspected bombers as the manhunt was under way.

The three - two students from Kazakhstan and a US citizen - were described as college friends of surviving bombing suspect Dzhokhar Tsarnaev, 19. They were not charged with direct involvement in the April 15 marathon bombings, which killed three people and injured 264.

But three days after the blasts, the trio moved swiftly to cover up for their friend when the FBI released pictures of the suspected bombers, made a public plea for help locating them and conducted a daylong manhunt that left much of Boston on lockdown, according to court papers.

Authorities charged the two Kazakhs, Azamat Tazhayakov and Dias Kadyrbayev, both 19, with conspiring to obstruct justice by disposing of a backpack containing fireworks they found in Tsarnaev's dormitory room. The third man, Robel Phillipos, also 19, was charged with making false statements to investigators.

After the proceeding, an attorney for Kadyrbayev denied wrongdoing.

Kadyrbayev and Tazhayakov face up to five years in prison, and Phillipos faces a maximum sentence of eight years.

Cover-up allegation

Kadyrbayev, Tazhayakov and Phillipos were placed in the custody of US Marshals after prosecutor Stephanie Siegmann argued that all three presented a "serious risk of flight".

Kadyrbayev and Tazhayakov had entered the US on student visas and lived in New Bedford, Massachusetts, according to the court papers. Phillipos is a resident of Cambridge, Massachusetts.

A month before the bombings, Dzhokhar Tsarnaev told Kadyrbayev and Tazhayakov over a meal that he knew how to make a bomb, Tazhayakov told the FBI, according to the documents.

On April 18, three days after the bombings, authorities released pictures of two men they identified as the suspects in the attack. Investigators at the time said they did not know the suspects' names and called on the public for help in identifying them.

Dzhokhar's three classmates quickly figured out their friend was one of the suspects, according to court papers.

The trio spent some time watching movies and then discovered an emptied-out fireworks tube, according to court papers. That discovery scared Tazhayakov, who then began to believe that Tsarnaev was involved in the bombing, according to court papers.

They decided to remove the backpack, fireworks and a laptop computer to help their friend "avoid trouble", according to court papers.

More cameras

Police and politicians across the US are pointing to the surveillance video that was used to help identify the bombing suspects as a reason to get more cameras on their streets.

From Los Angeles to Philadelphia, efforts include trying to gain police access to cameras used to monitor traffic, expanding surveillance networks in some major cities and enabling officers to get regular access to security footage at businesses.

"Look, we don't want an occupied state. We want to be able to walk the good balance between freedom and security," said Los Angeles police Deputy Chief Michael Downing.

Reuters-AP