In the end, bombing suspect is done in by a flapping boat tarp

(CNN) -- Something didn't look right. David Henneberry had been cooped up for hours, stuck inside as police searched for a suspect in the Boston Marathon bombings. He ventured out for some fresh air after police lifted their order to stay indoors.

He looked at the boat parked in his yard.

What he saw brought an end to the authorities' sweeping manhunt.

"The tarp on his winterized boat had sort of been flapping in the wind," his stepson Robert Duffy told CNN's Piers Morgan.

Henneberry had tied down the covering on the vessel in his backyard so firmly that it had not budged all winter long, not even through this year's blizzards.

"He got closer and realized that one of the retention straps had literally been cut -- not chafed, not broken or unhooked," Duffy said. Henneberry saw something else -- something on the tarp that looked like blood.

The Watertown resident thought some animal had forced its way into his boat. Maybe it was a squirrel, he told his stepson.

Henneberry climbed a stepladder to look inside.

"He basically stuck his head under the tarp (and) noticed a pool of blood," Duffy said.

'Crumpled up in a ball'

It was dark under the tarpaulin, so the boat owner could only make out vague contours, "but he definitely noticed there was something crumpled up in a ball," the stepson said.

A pool of blood; a manhunt in Watertown; time to call 911.

Squad cars with lights flashing raced in and lined the streets. Officers fanned out around the house.

Police whisked the Henneberrys so hastily from harm's way and into a neighbor's house that the couple left their cell phones behind at home.

Frantic phone calls

Duffy said he tried frantically to call his mother and stepfather as he watched on TV while law officers unleashed a hailstorm of gunfire into the backyard.

"We only saw the bullets; we only heard that there was a man covered in blood in the boat."

His stepfather, Duffy thought.

He kept dialing.

The couple's landline gave him a constant busy signal.

"It was absolutely horrifying for approximately 40 minutes," Duffy said.

He drove over to a relative's house to see if the phone lines worked better from there. As soon as he arrived, his sister called him.

"Mom's called, they're safe," she comforted him.

"At that point, my tears of horror went to tears of joy," Duffy said.

An end to the hunt

Henneberry's call to police put an end to the hunt for the man suspected of planting one of two bombs that exploded near the finish line of the Boston Marathon, killing three and inflicting injuries on dozens more that are typical of wounds sustained in conflict regions such as Afghanistan and Iraq.

Before police stopped him, Dzhokar Tsarnaev, with his older brother, Tamerlan, allegedly shot a university policeman dead, hijacked a car at gunpoint, then fired upon and threw grenades at officers pursuing them.

Police shot and killed the brother, but Dzhokar Tsarnaev eluded them all day Friday -- until Henneberry's 911 call.

Before opening fire, authorities, using a bullhorn, called on the younger Tsarnaev to surrender.

A standoff followed. When it finally ended and police approached the boat, they were aware of the danger the armed man posed.

Unlike Henneberry, they used a robot to pull back the tarp over the boat.