PORTLAND – A Massachusetts man had been drinking all day and then downed a six-pack of beer shortly before crashing his high-powered speedboat into another boat last summer on a Maine lake, killing the two people on the other boat, a prosecutor said Tuesday.

But Robert LaPointe’s attorney told jurors on the first day of his trial that LaPointe was sober and that a blood sample indicating he was impaired was tainted after being left in a hot vehicle for more than 30 hours. The crash, he said, was a “tragic accident.”

Jurors will have to decide which account to believe during LaPointe’s manslaughter trial, which is expected to last up to two weeks in Cumberland County Superior Court.

After the crash, LaPointe told people that he had been drinking all day and was running his 32-foot cigarette boat at 45 mph when it ran over a 14-foot boat on Long Lake in Harrison, Assistant District Attorney Jennifer Norbert told jurors in her opening statement.

Witnesses will testify how they heard LaPointe’s 32-foot boat, No Patience, with its twin 435-horsepower engines, roar up the lake on the moonless night of Aug. 11, 2007, Norbert said.

“And they heard a thump,” Norbert said. “And it lasted just a split second. And in that split second the lives of Terry Raye Trott and Suzanne Groetzinger were taken.”

LaPointe, 39, of Medway, Mass., is charged with two counts of manslaughter in the deaths of Trott, 55, of Naples and Groetzinger, 44, of Berwick. He also is charged with two counts of aggravated operating under the influence and reckless conduct with a dangerous weapon.

LaPointe faces up to 30 years in prison on each manslaughter count, plus up to five years on each of the other charges. He also faces a separate civil trial in the deaths.

His defense team is led by J. Albert Johnson, a Boston attorney who once represented Patty Hearst.

In his opening statement, Johnson said witnesses would testify how LaPointe was sober during the day and at the time of the 9 p.m. crash.

“At no time did he exhibit any qualities of being under the influence, of being impaired,” Johnson said.

Norbert told jurors that LaPointe and a family friend, 19-year-old Nicole Randall, left Naples on LaPointe’s boat to travel to Randall’s family’s home for dinner.

As they left, LaPointe saw the smaller boat with Trott and Groetzinger also heading out of Naples. The lights on Trott’s boat weren’t on, prompting LaPointe to shout, “Hey, idiots. You have no lights,” Norbert said. Later, LaPointe’s boat ran over the smaller boat.

There will be conflicting testimony on whether Trott’s boat had its lights on at the time of the crash, but the light switch was in the “on” position when the boat later was located on the lake bottom, Norbert said.

After the crash, LaPointe and Randall swam to safety while their boat zoomed ashore and came to rest about 100 yards in the woods. The victims’ bodies were found three days later.

LaPointe was taken to a Bridgton hospital where he told a nurse he had been drinking all day, Norbert said. He then suggested to the nurse that she substitute a sample of her own blood for his for the blood-alcohol test, the prosecutor said.

The test results showed he had a blood-alcohol content of 0.11 percent. The legal limit for operating a motor vehicle in Maine is 0.08.

Johnson told jurors that the blood sample was tainted and that his client was in fact sober and operating his boat prudently. The blood sample, he said, was left in a heated vehicle for 34 hours before it was delivered to a laboratory for testing.

“Each and every element must be proven beyond a reasonable doubt,” he said.

Dozens of witnesses are expected to testify during the trial, which Norbert said could last up to two weeks.