- Security company G4S says it can't provide all the Olympics guards it should
- G4S has a $444 million contract with the government for Olympic security
- The government is calling in 3,500 military personnel to bolster security
- Athletes began arriving for the Olympics on Monday
London (CNN) -- British lawmakers are set to grill the head of the security company that announced last week it could not provide enough staff for the Olympic Games, forcing the government to call in 3,500 military personnel to help.
Nick Buckles, the chief executive of G4S security, faces the House of Commons Home Affairs committee, which is probing Olympic security.
The Games open in 10 days.
G4S has a £284 million ($444 million) government contract to provide 13,700 security guards for the Olympic Games, but only 4,000 guards are trained and ready, the committee says.
The staff were supposed to be doing tasks including venue perimeter security such as manning X-ray machines, searching people, searching vehicles, and operating closed-circuit television systems, G4S told CNN on Sunday.
London orchestra plays train sounds 
Getting to the Games 
London's transport ahead of the Games 
'The busiest airspace in the world' Home Secretary Theresa May, who is responsible for domestic security, was called to Parliament to answer questions from lawmakers on Monday after the fiasco.
She insisted that G4S actually had more than 20,000 accredited security staff, and that until last week it appeared they would have too many contractors rather than too few.
The Home Office told CNN earlier on Monday that the contractor was suffering from a software problem which meant they could not guarantee who would turn up where, and whether guards had the right training.
G4S did not respond immediately to a CNN request for comment on the accusation.
Share your Olympics sights and sounds
The Home Office also said that the extra immigration staff deployed at borders was properly trained, rejecting media reports to the contrary.
Athletes began arriving for the Games on Monday.
The Dutch women's beach volleyball team sailed through Heathrow airport as it arrived in London, but not every athlete had such a smooth arrival.
American hurdler Kerron Clement's bus got lost on the way from the airport to the Olympic village, he said, resulting in a four-hour-plus journey.
"Athletes are sleepy, hungry and need to pee. Could we get to the Olympic Village please," the world record holder said on Twitter on Monday. "Not a good first impression London."
The drive should take about an hour.
Olympic athletes begin arriving in London 
How UPS handles Olympic size logistics 
Teen table tennis whiz competes for U.S. The London Organizing Committee of the Olympic and Paralympic Games (LOCOG) said "the vast majority" of bus journeys had gone smoothly, but conceded that "there may have been one or two journeys taking longer than expected."
Heathrow airport expected a record number of passengers on Monday as athletes began to flood into the city for the Olympic Games, which start a week from Friday.
Immigration desks have extra staff, Heathrow said Monday, amid fears of long lines to get into the country.
Retired border officials and retired police officers are among those being brought in to supplement immigration staff, the Home Office said.
Athlete arrivals are expected to peak on July 24, with more than 1,200 competitors due on that day.
See how Heathrow is preparing for the London Olympics
G4S said Saturday it stands to lose up to $77 million after failing to recruit enough staff.
The airport, meanwhile, said Monday it is deploying more than 500 volunteers who speak 20 languages among them to welcome athletes and officials.
CNN's Jo Shelley, Jim Boulden, Dan Rivers, Stephanie Halasz and Erin McLaughlin contributed to this report.
