Washington: Security was stepped up at the US airports and transit systems even as authorities said they have not found any plan for an attack similar to Tuesday’s bombings at Zaventem airport and a Metro station in Brussels. Department of Homeland Security Secretary Jeh Johnson said on Tuesday that no credible or specific intelligence regarding a similar plot has been uncovered, the Washington Times reported. "That said, we remain very focused on the threat posed by lone terrorist actors who may lack direct connection to a foreign terrorist organisation," Johnson said. Despite the explosions that rocked the Belgian capital, Homeland Security officials said they were not issuing any new terror intelligence bulletins or revising the terror alert level in the US. At least 34 people were killed and over 200 injured in a series of explosions at the main airport and in a Metro station of Brussels on Tuesday.