Less than a month after former NASCAR driver Greg Biffle and his family died in a plane crash last year, investigators say two of their “friends” conspired to break into the empty home and took cash, guns and financial information in an attempt to steal hundreds of thousands of dollars. More than 40 search warrants have been issued, authorities in North Carolina say, focusing on a married couple who allegedly knew Biffle and his wife Cristina. The suspects did “a lot of planning in an attempt to make a financial gain” off their deaths, Iredell County Sheriff Darren Campbell said. The Associated Press is not naming the pair because no arrests have been made. Biffle, his wife and his two children, along with three others, died in the Dec. 18 crash, which remains under investigation by the NTSB . Some survivors of those killed are suing the estates of Biffle and the pilot for millions of dollars.…