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