The man arrested last week in a double murder in St. John the Baptist Parish could not have pulled the trigger because he was at Harrah’s New Orleans Casino when the deadly shooting occurred, his attorney said Monday.

The attorney, David Belfield, said surveillance footage shows his client, Derrence Greenup, at the casino during the early hours of Oct. 5 — an alibi he described as airtight.

Belfield accused Sheriff Mike Tregre of arresting Greenup in haste to score political points ahead of this month’s election.

“He knows that the gentleman was at Harrah’s Casino at the time of the murder — there’s no doubt about that,” Belfield said in a telephone interview. “This is basic ‘Oops, we got the wrong person’ kind of policing.”

Tregre declined to respond to Belfield’s allegations. He confirmed that Greenup has offered an alibi, which he said remains under investigation.

“All that’s being done as we speak,” the sheriff said. “We are working the case.”

Greenup, 28, of Reserve, was jailed Thursday in connection with a shooting in a quiet subdivision in LaPlace that took the lives of cousins Travis and Sierra Gregoire and wounded a 23-year-old man. The shooting happened about 2 a.m. after the victims pulled up in a car in the 400 block of State Street.

The authorities have said a white pickup believed to have been used in the killings was found burning in Reserve a day later.

Greenup, who has a history of prior arrests, was taken into custody in a rental car lot near Louis Armstrong International Airport. He faces two counts of first-degree murder and one count of attempted first-degree murder.

Tregre has declined to say how investigators connected Greenup to the crime, noting that two alleged accomplices remain at large. A motive for the shooting also remains unclear.

The sheriff offered no new details Monday, saying in a text message that the Sheriff’s Office “obtained evidence that gave (it) probable cause which led to the arrest of Greenup.”

Belfield said he has “no clue” what that evidence might be.

Greenup had “no beef” with the victims and did not even know them, Belfield said. On the night of the shooting, he added, Greenup went to the Saints game and then the casino.

“His vehicle didn’t leave Orleans Parish that night,” Belfield said. “He may not be an altar boy, but he didn’t do this.”

Belfield also accused the sheriff of improperly targeting a St. John criminal defense and civil rights attorney, Nghana Lewis Gauff, who owns a rental property in New Orleans that investigators searched Friday.

Tregre said authorities are investigating whether an AK-47 found in the residence is related to the fatal shooting. He said Greenup will face an additional count of possessing a firearm as a convicted felon.

“We had reason to believe that Greenup had established — I don’t want to say domicile there — but he did have access to this home,” Tregre said. “Once we found that he did have access, we obtained a search warrant ... and recovered an AK-47.”

Gauff, who has previously represented Greenup, declined to comment Monday.

Belfield, however, said he believed Tregre was retaliating against Gauff for filing a series of civil rights complaints against his deputies.

“Nghana Gauff has, I think, three or four cases pending against the sheriff right now for stupidity, for lack of a better word,” Belfield said. “Here’s his chance to strike back at her simply because he found a weapon in a house that she owns that was being rented out. Ms. Gauff lives in LaPlace. The house they searched is on Broad Street” in New Orleans.

Follow Jim Mustian on Twitter, @JimMustian.