liaison Marines called in air and artillery strikes throughout the day. Lieutenant Colonel Richard Barry, commander of the forward headquarters of the 1st Surveillance, Reconnaissance and Intelligence Group, sent warnings about an impending attack to Central Command. CentCom leaders were too preoccupied with the air campaign to heed them however, and so the Iraqi operation came as a surprise. Beginning of Iraqi offensive: 29 January The Iraqi offensive began on the night of 29 January, when approximately 2,000 soldiers in several hundred armored fighting vehicles moved south. Post-war analysis by the US Air Force's Air University suggests Iraq planned to utilize the 3rd Armored Division and 5th Mechanized Division to make the actual attack on Khafji, with the 1st Mechanized Division assigned to protect the attacking force's western flank. The Iraqi incursion into Saudi Arabia consisted of three columns, mostly made up of T-62 tanks and armored personnel carriers (APCs). The Gulf War's first ground engagement was near observation post 4 (OP-4), built on the Al-Zabr police building. Elements of the Iraqi 6th Armored Brigade, ordered to take the heights above Al-Zabr, engaged Coalition units at Al-Zabr. At 20:00 hours, U.S. Marines at the observation post, who had noticed large groups of armored vehicles through their night vision devices, attempted to talk to battalion headquarters but received no response. Since contact earlier was no problem, there was a strong presumption that the reconnaissance platoon's radios were being jammed. Using runners, Lieutenant Ross alerted his platoon and continued trying to get through and inform higher headquarters and Company D of the oncoming Iraqi force. Contact was not established until 20:30 hours, which prompted Task Force Shepard to respond to the threat. Coalition soldiers at observation post 4 were lightly armed, and could only respond with TOW anti-tank missiles before calling in air support. Air support arrived by 21:30 and took the form of several F-15E, F-16C, four A-10 Tank Killers and three AC-130 gunships, which intervened in a heavy firefight between Iraqi and Coalition ground forces at OP-4. The reconnaissance platoon stationed at OP-4 was the first to come under attack, their withdrawal from the engagement was facilitated by another company providing cover fire. The attempt by the soldiers stationed at OP-4 to fend off or delay the Iraqi advance cost them several casualties, and in the face of a heavy Iraqi response they were forced to retire south, by order