Insurgents used "sophisticated" weaponry to shoot down a US Marine transport helicopter was that crashed last week northwest of Baghdad, a US military spokesman said Wednesday. Pentagon officials initially blamed mechanical problems for the crash of the CH-46 Sea Knight, which went down Feb. 7, killing all seven service members aboard. On Tuesday, the US Marines said further investigation confirmed the helicopter was brought down by hostile fire. "It was probably brought down by some sophisticated piece of weaponry," chief military spokesman Maj. Gen. William Caldwell told reporters. Caldwell did not elaborate and did link the incident to weapons which the military says have been smuggled to Shiite extremists from Iran. The Sea Knight crashed in an area where Sunni insurgents operate. At least seven US helicopters have crashed or been forced down by hostile fire since Jan. 20. US officials have said extremists had used small arms against them, and Caldwell's statement was the first time a senior officer has spoken of sophisticated weapons, which could include shoulder-fired anti-aircraft missiles.