AINTREE, England (AP) — The Grand National races are under scrutiny after four horses died in the past year at the event.

The world's most grueling steeplechase begins on Saturday, and there's concern about the safety of the horses after two died in each of the last two races.

The National — a 4 1/2-mile, 30-fence race — remains one of the fixtures on the British sporting calendar, attracting millions of television viewers and visits to bookmakers each year. Past winners, such as three-time champion Red Rum, have been adopted as national treasures.

One euthanized horse was Synchronised, the favorite and Cheltenham Gold Cup champion at the time.

Recent modifications to the course have failed to satisfy critics and animal rights groups who call for the event to be scrapped. Ten horses have died in the Grand National in the past 12 years, and 20 have died in races over Grand National fences since 2001.

"I hope to God there are no accidents this year, but these things happen," jockey Katie Walsh said.

Jonjo O'Neill, the trainer of Synchronised, agrees.

"It's always been a fantastic race and it still is," O'Neill told The Times of London. "People saying the National should be stopped just don't understand. Some of the things said about us are an insult, as if we are animals ourselves. There are people out there stabbing and shooting, killing randomly, and sometimes it seems we are being put in the same bracket."

The severe injury suffered recently by JT McNamara, who was paralyzed after falling from his horse during the Cheltenham Festival last month, has also increased concerns about the safety of jockeys. Many of the famously high fences involve sheer drops.

Organizers are faced with balancing the race's enduring appeal and attempting to keep it as risk-free as possible. They settled on changes to fence design, landing areas and course irrigation following a review in May.

The start has been brought 90 yards forward, away from the noise of the crowd in the grandstands, after issues in the 2012 race and in 1993 — when the race ended up being declared void.

There will still be 40 horses involved, despite criticism of a congested field leveled at the National. Becher's Brook — the much-feared fence — also keeps the same drop and dimensions.

Usually one of the fiercest critics of the race, the Royal Society for the Prevention of Cruelty to Animals, has been impressed by the changes.

"In all honesty they have done more than I thought they would," said David Muir, the RSPCA's equine consultant. "Fundamentally, the changes that have been made are major already.

"They've taken the cores of the fences out, there is a cooling-down area now, there's a water system and there's a reduction in the number of drop fences. We'd still like to see changes to Becher's Brook, the drop is still a concern."

More fatalities this weekend would leave the event open to more criticism.

"As a team we're going in very positive and with confidence in the race, its history and its tradition," said John Baker, who runs the Aintree course in Liverpool. "By anyone's standards, there was bad luck involved last year."

Unlike last year, the Cheltenham Gold Cup winner won't be going for the distinguished steeplechase double, not achieved since 1934, because Bobs Worth hasn't entered.

Reigning champion Neptune Collonges has retired, leaving only one previous winner in the field — 2011 champion Ballabriggs.

The early favorite with British bookmakers is Willie Mullins-trained On His Own (15-2) while Seabass and Cappa Bleu, who were third and fourth last year, are heavily backed.

Imperial Commander, the 2010 Gold Cup winner, has entered and is top weight.