UPDATE 1-U.S. Attorney expects indictments in U.S. Capitol breach as soon as this week

(Updates with details from press conference)

By Sarah N. Lynch and Doina Chiacu

WASHINGTON, Jan 26 (Reuters) - The top federal prosecutor investigating the violent breach of the U.S. Capitol said on Tuesday he expects indictments will be handed down as soon as this week, as the FBI works to track down more than 400 suspects in the sprawling investigation.

More than 135 people have been arrested in connection with the Jan. 6 breach, which left five people dead and sent members of Congress rushing to safety, Acting U.S. Attorney Michael Sherwin told a news briefing.

The vast majority of the suspects to date were arrested on criminal complaints, which serve as a placeholder that allow the government to build a criminal case until it can be presented to a grand jury. A person cannot be convicted solely on a criminal complaint in federal court.

To date, only a handful of indictments have been returned against the more than 100 people charged with a range of crimes, from minor trespassing offenses to more serious allegations such as assault on police officers and conspiracy charges.

"We're continuing to build the evidence related to these charges," Sherwin said. "The individuals that were charged via the federal criminal complaints will then be indicted in the very near future to include this week."

The FBI is investigating whether groups of people may have plotted in advance to storm the Capitol, and Sherwin said on Tuesday he expects some will be facing seditious conspiracy charges.

Earlier this month, his office charged three members of the far-right anti-government militia known as Oath Keepers with conspiring ahead of the Capitol siege.

No indictment has been made public yet in that case.

Sherwin said the investigation is starting to turn towards more deep dive investigations given that the FBI has already managed to track down many of the "Internet stars" who bragged about their actions on social media.

"We are going to reach a plateau," he said. "The plateau will involve ... looking at the more complicated conspiracy cases."

Federal officials also said on Tuesday the investigation into the death of Capitol Police office Brian Sicknick and the planting of two pipe bombs at the Democratic and Republican headquarters offices is still going on. (Reporting by Sarah N. Lynch and Doina Chiacu; additional reporting by Mark Hosenball; Editing by Franklin Paul and Sonya Hepinstall)