Fowler is bidding to become the seventh first-time major victor in a row with in Wisconsin this week.
Casey was joined at 66 by 23-year-old American Xander Schauffele.
According to a tweet by the organizers of the US Open, USGA, the commercial blimp was not linked with the U.S. Open or with sponsors FOX Sports.
Talor Gooch, Thomas Aiken and Tommy Fleetwood are all at 2 under early in their rounds - one shot behind Fowler.
And I tend to play better on this type of golf course, so you enjoy what you play well on".
"So a little bit of timing, and a bit of rust as well". "But towards the end I holed some great putts for par". "Obviously, I paid the price for it today". Patrick Reed is 4-under after shooting 68.
Fowler had seven birdies and no bogeys. "No matter how fast you start or how many birdies you make the pars are the ones that keep you in it". He settled in nicely, though, recording six birdies and two bogeys.
"I just didn't putt very well", Johnson said afterwards. "But there's way too many holes out here where one bad shot could be a double bogey quickly".
Yesterday's opening round was overshadowed by the fiery crash of a blimp hovering near the course.
Fans and media have reported seeing smoke and fire coming from the blimp while hovering in the air before crashing.
Twitter users captured some of the crash, adding that they saw a person evacuating with a parachute. Lee Westwood, Jamie Lovemark and J.B. Holmes are all 3-under with 69s.
There were as many amateurs who broke par as players in the top 10 in the world - two each.
Spanish star Garcia opened with an eagle three on the 613-yard par-five first while McIlroy also grabbed an early eagle on the par-four second.
The four-day tournament started Thursday. Martin Laird carded a level-par 72, while fellow Scot Russell Knox shot a 73 to sit in a group which included 2015 champion Jordan Spieth.
Not only is it hard to find the ball without a spotter or a caddie, it is tough for a golfer of any caliber to have enough clubhead speed to get through the fescue and even hit the ball.
But world number one Dustin Johnson - the defending champion and pre-tournament favourite - was struggling to get to grips with his game.
Johnson said he wished conditions were more challenging. He will have to do some serious work over the next two days to get back in the mix.