Washington, Feb 10 (IANS): The police in Washington D.C. said that they were "working to thoroughly investigate" bomb threats made to eight schools around the US capital.

The public schools and charter schools had all been cleared with no hazardous material found, the Metropolitan Police Department tweeted late Wednesday.

The investigation is underway with the help of federal agencies.

Several D.C. schools, including Dunbar High School, were evacuated on Wednesday afternoon after Second Gentleman Doug Emhoff was escorted out of an event at Dunbar High School on Tuesday due to a bomb threat.

"These are troublesome incidents that we take very seriously," Lewis D. Ferebee, chancellor of D.C. Public Schools, tweeted on Wednesday.

More than a dozen historically black colleges and universities nationwide also received bomb threats in the early days of this month dedicated to honoring achievements and struggles of African-Americans throughout US history.