NEW YORK (CNN) -- A helium-filled balloon taking part in New York's annual Thanksgiving parade hit a lamppost near Times Square on Thursday, causing a light to fall and injure two sisters from Albany, officials said.An 11-year-old girl suffered a head injury and her 26-year-old sister, who uses a wheelchair, suffered a shoulder injury, police said. Both were taken to Bellevue Hospital in Manhattan.There, the two were treated for minor head injuries and released, hospital spokeswoman Kate McGrath said.During a press conference after the event, Mayor Michael Bloomberg said the 11-year-old suffered a minor abrasion, and her sister required six stitches.The accident happened around 11:30 a.m., toward the end of the parade route. A police spokesman said a cable from the M&M's candy balloon became entangled with the lamppost. (Watch a spectator's video of the mishap -- 2:01)Before the parade, city officials considered -- and then decided against -- grounding parade balloons because of forecasts of strong wind. Mayor Bloomberg said the cause of the accident would be investigated, but a gust of wind may have been to blame.Winds "were well within the limits," Bloomberg said. They were gauged at 15 mph at the time, safely below the 23 mph limit for parade balloons to fly.During the 1997 Macy's parade, heavy winds caused The Cat in the Hat balloon to strike a lamppost, which toppled onto four people, leaving one woman with permanent brain damage.Since then, Macy's has implemented improved training techniques for the approximately 1,700 people who help handle and direct the massive balloons."The training is more extensive, the training has been upgraded, the training is very successful, and the training is monitored by the police department of New York City," said John Piper, vice president of Macy's Parade Studio, before Thursday's event.The M&M's balloon was 40 feet long, 30 feet wide and 50 feet tall and weighed 515 pounds when fully inflated with helium. It was towed by 48 handlers, according to Macy's."Only two of these people were doing this for the first time," said parade executive producer Robin Hall.Aerial video 20 minutes after the accident showed a broken lamppost light lying on a sidewalk and volunteers rolling up a deflated balloon.