The defendant, Preston Thissell, appeals his conviction, following a jury trial in superior court, on a charge of negligent homicide. See RSA 630:3 (2007). He argues that the trial court erred by: (1) denying his motion for a mistrial following a violation of a sequestration order by two of the State's witnesses; and (2) excluding testimony from a certain witness. We affirm.
We first address whether the trial court erred by denying the defendant's motion for a mistrial. "A mistrial is appropriate only if the evidence or comment complained of was not merely improper, but also so prejudicial that it constituted an irreparable injustice that cannot be cured by jury instructions." State v. Russo, 164 N.H. 585, 589 (2013) (quotation omitted). Because the trial court is in the best position to gauge the prejudicial impact of events at trial, it has broad discretion to determine whether a mistrial is warranted, and we will not overturn its decision absent an unsustainable exercise of discretion. Id. To establish an unsustainable exercise of discretion, the defendant must show that the trial court's ruling was clearly untenable or unreasonable to the prejudice of his case. State v. Perri, 164 N.H. 400, 408 (2012).
The evidence in this case establishes that the victim died in a "head-on" automobile collision. At trial, the victim's girlfriend testified that she, the victim, and the defendant were all acquaintances, that she and the victim had been involved in a dispute with the defendant in the twenty-four hours prior to the crash, and that immediately prior to the crash, the defendant was: (1) following the victim's car so closely that the victim could not see the defendant's headlights; (2) beeping his horn; (3) pulling up beside the victim's car in the lane of oncoming traffic; (4) swerving toward the victim's driver's-side door; (5) motioning for the victim to pull over; and (6) revving his engine as if to pass the victim. The crash occurred when the victim sped up and lost control of his car.
At trial, Sergeant Richard Chambers of the Hampstead Police Department testified that, in the aftermath of the collision, he requested that a New Hampshire State Police Traffic Accident Reconstruction (TAR) team perform an "accident reconstruction." On cross-examination, however, he agreed that in his report, he stated only that he had requested "a diagram and measurements, " not an "accident reconstruction." Nevertheless, he continued to maintain that he requested, and that the TAR team performed, an accident reconstruction.
Following Sergeant Chambers's testimony, counsel for the defendant notified the trial court that Sergeant Chambers had been seen conversing with Trooper Brian Strong, a witness for the State who was part of the TAR team, and who had not yet testified. After conducting a voir dire examination of both witnesses, the trial court concluded that the conversation concerned Sergeant Chambers's testimony and violated a sequestration order. The defendant requested a mistrial, arguing that his cross-examination of Trooper Strong had been compromised, and that a jury instruction could not cure the prejudice. The trial court denied the request for a mistrial, ruling instead that it would allow the defendant to cross-examine Trooper Strong about the conversation, and that it would instruct the jury that the conversation violated the sequestration order.
Trooper Strong testified that his role in the investigation "was very limited." He explained that the TAR team was asked to diagram and document the scene. According to Trooper Strong, the TAR team walked the crash site, took photographs and measurements, and created a diagram. He agreed with the defendant that he had neither conducted, nor been asked to conduct, a "full accident reconstruction."
Prior to Trooper Strong's cross-examination, the trial court instructed the jury as follows:
Before trial, the Court ordered that witnesses be sequestered. This means that a witness cannot discuss the content of his testimony with another witness until both of them have testified. The purpose of this order, basically, is to ensure that a witness does not influence the manner [in] which another witness testifies.
In this case, the Court made a determination that there was a violation of the sequestration order when Sergeant Chambers talked to Trooper Strong about Sergeant Chamber[s's] testimony before Trooper Strong had a chance to testify, and you may give this information any weight that you think it deserves.
On both direct and cross-examination, Trooper Strong testified concerning his conversation with Sergeant Chambers, and he admitted that it was improper.
On appeal, the defendant argues that the trial court violated his rights to a fair trial under the State and Federal Constitutions. He further argues that the sequestration order violation tainted the judicial process with an appearance of impropriety, and that the testimony of both Sergeant Chambers and Trooper Strong should have been excluded. The record reflects, however, that the defendant did not raise these arguments at trial. Accordingly, these arguments are not preserved, see State v. Noucas, 165 N.H. 146, 152 (2013), and we confine our review to whether the trial court unsustainably exercised its discretion by denying the request for a mistrial, Russo, 164 N.H. at 589.
Mistrial is an extraordinary sanction, which should be granted sparingly. State v. Bertrand, 133 N.H. 843, 854 (1991). We have noted that "[t]he accepted practice . . . requires the trial court, prior to declaring a mistrial [for violating a sequestration order], to exhaust alternatives, including giving permission to recall witnesses and giving cautionary instructions." Id. "Curative instructions are presumed to be understood and followed by the jury." Id.
In this case, the record reflects that Trooper Strong contradicted Sergeant Chambers and supported the defendant's position concerning the extent of the investigation the TAR team was asked to perform. Moreover, the trial court specifically instructed the jury that Trooper Strong and Sergeant Chambers had violated the sequestration order, that the order was in place to assure that the witnesses did not improperly influence the testimony of each other, and that the jury was free to evaluate the witnesses' testimony in light of the violation. Under the circumstances of the case, we cannot say that the trial court's determination that a mistrial was not warranted was either clearly untenable or unreasonable to the prejudice of the defendant's case. Perri, 164 N.H. at 408.
We next address whether the trial court erred by excluding testimony from a certain witness. Whether to exclude evidence at trial is a matter left to the sound discretion of the trial court. State v. Furgal, 164 N.H. 430, 438 (2012). In evaluating the trial court's exercise of discretion, we consider whether the record establishes an objective basis to sustain the decision. Id. To establish that the trial court unsustainably exercised its discretion, the defendant bears the burden ...